One of the peculiar points about a Starshards oriented Priest build is that neither Starshards itself, nor Shadow Word: Pain for that matter, can critically hit … since they’re both DoT spells (one channeled, the other instant). Your other Holy and Shadow damage and healing spells can critically hit though, so it makes sense to invest in raising critical hit chances for your Holy spells at least. But in a Starshards build, you’re rarely going to be using Smite, except perhaps as a “filler” spell or those times when you combine it with Mind Blast for a particularly fast damage spike or an opening alpha strike while stuff is bumrushing towards you.
Instead, the primary spell rotation involves using Holy Fire, Starshards (melee swing, optional) … repeat. This is then a 9.0-9.5 second spell rotation (when not including a melee swing) that makes optimal use of the DoT burn of Holy Fire (best results when including a melee swing) while simultaneously leveraging mana recovery from Spirit in an extremely efficient way (MINIMUM 50% mana recovery from Spirit while continuously casting, up to 80%-ish(!) depending on situational factors, talents and gear).
Now with the benefit of experience, I can say that investing in Divine Fury and Searing Light does help with the Holy Fire+Starshards+melee swing spell rotation more than I was expecting it to. Even though Searing Light only adds an extra +10% to one of the two main spells you’re casting (Holy Fire), it also boosts Smite, and on more than one occasion that extra +10% damage on those Holy spells has meant the difference between needing to cast an extra spell or not. Reducing the number of spells cast to kill by -1 is helpful not only in saving time, but also in saving mana (for that engagement). However, that won’t always be the case, so Divine Fury+Searing Light falls into the Nice To Have™ category, rather than being a MUST HAVE AT ALL COSTS! type of deal.
A side benefit of that investment into Divine Fury however is that you make a decent “off main” or solo healer, when the circumstances call for it. Indeed, relative to “main” healers, you aren’t going to be that short on healing power from Holy, by comparison in talents. The main difference is that you won’t have Holy Reach (for extra range), Spiritual Healing (for +10% more healing), or Spirit of Redemption 
, following the build strategy I’ve laid out. However the big thing you’d be short on is that as a Damage Per Mana (DPM) caster, if you gear for spellpower that’s good for All Schools (and healing), you’ll have less spellpower than if you were specializing in just healing cloth gear. So not a “main” healer, but you can certainly pick up the slack as an “off main” healer if need be … which is just fine with me, since I would rather blast away as a damage dealer than get stuck “flying the gauges” as a healer anyway. But the swing role versatility is there, inside the build.
=====
The thing that turns this build into an orbital artillery cannon is the sheer amount of Spirit that it can stack up AND MAKE USE OF while continuously casting. Spirit Tap DOUBLES your Spirit for 15s … and yes, that includes +Spirit mods from equipped items. Divine Spirit (Rank 4) increases your Spirit and can be cast on yourself. Spiritual Guidance (Rank 5/5) gives you +1 Spellpower (all schools) for every 4 Spirit you have (drop fractions). If you have 400 Spirit, you’ve added +100 Spellpower to everything you can cast. All of which means that for this build Spirit is NOT your dump stat …! On the contrary, given how much mana you can recover from Spirit while continuously casting, piling on more and more Spirit onto this build not only “deepens” your mana pool (in effect) it also increases how powerful your spells are at the same time … win-Win-WIN!
Having played this build on both Darrowshire (Elysium) and Symmetry, I can categorically say that stacking as much Spirit as possible onto this build strategy can be rather … 
eye watering … is the most expressive way to put it. I was walking into Scarlet Monastery and Razorfen Downs with more than +100 total Spellpower @ Level 39 thanks to all the Spirit itemization I had equipped on top of the usual +Spellpower cloth pieces I’d tailored for myself, and I was taking full advantage of it (100% spellpower coefficients). It was glorious. 
How glorious? 
Let’s just say that it felt like I had a BOTTOMLESS MANA POOL … except when I had to recast Power Word: Fortitude on dungeon parties. I’d usually volunteer to do that while letting the other Priest handle Healbot Duties (so they could better manage their mana during dungeon runs).
Fun part was telling the other casters in dungeon runs to check my mana bar in order to do BEFORE and AFTER comparisons around combat encounters. They could see that I wasn’t stopping to drink (like a Mage), even after a heavy pull and a hard fight with plenty of adds (because … SPIRIT!). If I started a fight at full, by the end of that fight (while casting almost continuously) I’d be maybe 25-30% down from max on my mana bar, because my build was SO GOOD at net mana cost/recovery balancing while fighting. I’d be “keeping up with the Mage” on Damage output, but my net mana balance was SO EFFICIENT that I was producing all of that damage at barely a net loss in mana over time, meaning my ENDURANCE was exceptional as a damage caster. The other casters could barely believe what they were seeing happen right in front of them, in real time, as I fought alongside them. 
And then when each combat was over … my mana bar refilled back up to full in ~10-15 seconds, while the Mage (and other casters) were still sitting down and drinking. The people I was partied up with had never seen anything like it before. 
- First they laughed, when I told them I was playing a DPM (a what?
) Priest build (but wasn’t Shadow!). 
- Then they SAW what a Damage Per Mana Priest COULD DO …
- And then they started asking questions, that were difficult to answer in chat, because explaining took too long (and I kind of needed to use ASCII Art to get the idea across).
=====
So the way this “works” is by abusing skillfully designing around the way the 5 Second Rule of Casting functions by carefully “shaping” (Art of TETRIS style) the rotation of what spells you cast (when and why). 
What is ACTUALLY HAPPENING (game mechanically, under the hood) is that every 2 seconds, if your class has a mana bar, you get a mana recovery tick (from Spirit). The amount of mana that gets added to your mana bar can be adjusted, but the timing of the recovery ticks does not.
When you cast a spell that 
costs mana 
(meaning, more than zero!), a DEBUFF gets automatically (and silently, meaning there’s no debuff icon for you to see) applied to you for 5 seconds. That debuff is a -100% to the mana recovery you get from Spirit while the Debuff is active. This Debuff is what produces the 5 Second Rule of Casting.
If a spell costs zero mana to cast (because of Clearcasting, or whatever), the 5 Second Rule of Casting Debuff is not invoked/applied, so mana recovery from Spirit does not slow/stop.
The min/max crowd will already be noticing that if mana recovery from Spirit happens every 2 seconds … and the 5 Second Rule of Casting Debuff lasts 5 seconds … then logically, that means … that after casting a spell that costs mana, either 2x mana recovery ticks (4 seconds worth) or 3x mana recovery ticks (6 seconds worth) will be impacted depending on the timing of those mana recovery ticks relative to when the mana consuming spell was cast. Basically, does the “last 1 second” of the Debuff “overhang long enough” to affect another mana recovery tick before the Debuff expires? The answer to that question gets “more complicated than you’d expect” on a case by case basis, mainly because by that point you’re talking about timing factors of less than 1 second (which can be hard to control for when network and server lag times are variable, not to mention how “twitchy” Player reaction times can potentially be).
Now, where this factor becomes important for the “shaping” of spellcasting is … WHEN different spells deduct their mana cost … which then invokes the 5 Second Rule of Casting Debuff. In a text forum like this one, the easiest way to represent this is with a symbol. For the purposes of this explanation, I’ll use: 
So when you cast a spell that is either Instant or has a Cast Time on it, the mana cost comes at the “end” of the casting (so if there’s an interruption of any kind, no mana is spent). The easy way to think of how that works in practice is like this:
- Casting Time
For channeled spells though, this is reversed. The mana cost comes at the “front” of the channel (so if there’s an interruption of any kind, too bad/so sad, you’ve already spent your mana but you aren’t getting the full benefit of that expense). The easy way to think of how that works in practice is like this:
- Channeling Time
Where this all comes together as an unrealized advantage for classes that have both Cast Time+Channeled spells is when you can “shape” your attack rotation around this factor of how the 5 Second Rule of Casting ACTUALLY WORKS in practice.
- Casting Time Channeling Time = 6+ seconds (the higher the better!)
Because …
- Casting Time Channeling Time . Casting Time Channeling Time . (repeat)
The “longer” you can make the Casting Time+Channeling Time in the rotation, the more mana recovery ticks from Spirit you’ll be able to obtain/capture WHILE continuously spellcasting.
Starshards Priest (at range)
- Holy Fire (3.0-3.5s) Starshards (6s) . (repeat) = 9.0-9.5s spell rotation
What this means in practice is that (worst case scenario) the 5 Second Rule of Casting Debuff EXPIRES at 5 out of 6 seconds of channeling time during Starshards … therefore … every time that Starshards can be channeled to completion, mana recovery ticks from Spirit are “operating” at 100% mana recovery rates. That means that for the next 3.0-3.5 seconds AFTER the Starshards channel runs the full 6 seconds until the next Holy Fire casting, you’ll be earning 2-3 mana recovery ticks from Spirit … even though you’re already in the process of casting the NEXT Holy Fire.
Starshards Priest (in melee)
- Holy Fire (3.0-3.5s) Starshards (6s) . Melee Swing (1.5s+) . (repeat) = 10.5s+ spell+swing rotation
By extending the spell rotation even longer (thanks to the addition of the melee swing), you can basically add +1 (or more) mana recovery ticks from Spirit into the mix for reduced “net mana loss” per rotation. Even better yet, Auto Attack for melee weapons (but not Shoot for Wands) can be cued WHILE CHANNELING without interrupting the channeling, which will start the swing timer on your weapon while the channeling is ongoing (and it COUNTS!) so that when the channeling ends you swing IMMEDIATELY instead of waiting for the swing timer to add up and release a swing. 
Starshards Priest (opening from maximum range) (used only when needed)
- Holy Fire (3.0-3.5s) Instant Cast (0s) Starshards (6s) = 10.5-11.0s spell rotation
This is what I do to initiate combat when solo. I cast Power World: Shield after Holy Fire and before Starshards. This allows me to start at 36ft range with Holy Fire and my target will be within 30ft by the time I need to start channeling Starshards (because there’s no range enhancement via talents for Starshards, so it’s 30ft range forever). This also means that a portion of the 15s debuff before Power Word: Shield can be recast (on myself) will have elapsed by the time my target bumrushes my location to start melee engaging my Priest, meaning taking less damage while the debuff preventing recasting of the bubble (making it easier/safer to survive if the situation Goes Bad™). Likewise, if my Power Word: Shield bubble gets broken and I need to recast it, it’s better (net mana management-wise) to do that recasting “in between” a Cast Time spell and a Channeled Time spell, so as to “sandwich” the mana costs in ways that prevent the 5 Second Rule of Casting Debuff from “stealing” too many mana recovery from Spirit tick opportunities from me.
And just to lay all the cards on the table about the 5 Second Rule of Casting Debuff … various talents and items/set bonuses will MODIFY the value of the Debuff away from its default of -100% for that character. So a talent like Spirit Tap will allow 50% of mana regeneration from Spirit while casting during the time that the Spirit Tap buff is active. What it’s doing is modifying the -100% mana recovery from Spirit Debuff from the 5 Second Rule of Casting into being only a -50% mana recovery because of Spirit Tap … allowing “half” (of the value) of your mana recovery tick values to be operational and add to your mana pool. The mana recovery ticks NEVER STOP HAPPENING, but the “amount” of mana that they YIELD can be Debuffed down to nothing (so it looks like they’ve stopped in the UI). Same thing happens with Mage Armor and Meditation and other similar types of talents, items and effects.
=====
Note that the above “structuring” of Cast Time+Channeled Time is something that can be taken advantage of by additional classes, so this ISN’T something unique and exclusive to (Night Elf) Priests per se.
Mage
- Pyroblast (6s) Arcane Missiles (3-5s) . (repeat) = 9-11s spell rotation
- Fireball (3.0-3.5s) Arcane Missiles (3-5s) . (repeat) = 6.0-8.5s spell rotation
- Frostbolt (2.5-3.0s) Arcane Missiles (3-5s) . (repeat) = 5.5-8.0s spell rotation
Warlock
- Shadow Bolt (1.7-3.0s) Drain Life (5s) . (repeat) = 6.7-8.0s spell rotation
- Note: Lifetap and Dark Pact cost zero mana to cast!
Druid
- Starfire (3.0s-3.5s) Barkskin (0s) Hurricane or Tranquility (10s) = 14.5-15.0s spell rotation (not repeatable)
And yes … I found all of that out because I played a Starshards Priest for the first time almost a decade ago (and I’ve been using it to inform build choices for multiple characters in other classes ever since!). 
Offer Not Valid for PvP oriented playstyles (where channeling is a “kick me, bro!” sign).
=====
The sum total of all of these disparate parts would appear to create a most unusual combination … of an Arcane Night Elf Priest who is extremely mana efficient (!) at producing sustained damage pressure on a single target, and with some ability to apply damage to multiple targets simultaneously (via Holy Nova). The irony of course being that this is a Priest build instead of a Mage build, and Night Elves are hardly renowned for their … aptitude … for the Arcane, having turned away from their arcane studies after the Sundering in a bid for survival. 
And yet, here is a build that combines Arcane, Holy and Shadow together (21/25/5) in what feels (to me) like a rather viable balance in, of all things, a Priest … and more specifically, in a Priest dedicated to the Goddess Elune.
This is a talent build that (probably) only someone who allowed themselves to bathe in the light (and shadow) of Elune could love and follow in our world of Azeroth. Fortunately, that’s where still I am, standing for the Goddess in the place between the Shadow and the Light.
/pray
/cast Starshards
However, the discoveries that I made synergizing long Cast+Channel spell rotations is the cornerstone foundation for making this build one of (in not the most!) mana efficient damage producing builds I’ve ever encountered. Furthermore, the long Cast+Channel spell rotation “shape” of spellcasting has implications no only for this particular Starshards Priest build, but also for Arcane Missiles Mages and Drain Life Warlocks as well. Technically, Druids can (almost) do it too … just not repeatedly as part of an attack chain (because of the cooldown times on the Hurricane and Tranquility spells). More on this point of broader implications, later. 