This spec makes you durable against many classes that crush SP: war, dk, rog, feral.
The reason you are strong is because melee attacks trigger a Discipline talent called Focused Will, which reduces all damage you take. If you are standing in shadowform against a warrior that is attacking you, you will have Shadowform reducing damage by 15%, Focused Will reducing damage by 12%, Renewed Hope reducing damage by 3%. That is only talents, before your resilience comes into play. You play this spec on extremely high resilience.
With this spec. you have strong defense that does not require GCDs. You just reduce damage A LOT. This is complemented by putting talents into tankiness like better Power Word Shield, Power Word Fortitude, and glyphing Glyph of Power Word: Shield, and Glyph of Inner Fire. I play NE priest on 1600 resillience. Play 3/5 Mooncloth, 2/5 Satin. And Bauble trinket.
Your tankiness makes playstyle very easy, casual, and strong. You survive easily. You are not dependent on many cooldowns, and you have lots of little things that you can play around like Lifeblood, Power Infusion., and Bauble trinket.
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You are going to think the talent is bad because of Spirit Tap, however I assure you this spec is extremely strong, and Spirit Tap is used effectively.
Glyph of Shadow and Spirit Tap work together. (You increase your spirit significantly, and then that increases your SP)
Since your playstyle is to spam Mind Flay, you are always proccing Spirit Tap, and Glyph of Shadow. This gives you a constant source of mana regen, as well as bringing your spell power up to normal levels to compensate for your heavy resilience investment. After a killing blow, you have a massive SP bonus and mana regen. (Reflective Shield talent also crits enemies, proccing your Glyph of Shadow, so you really have it up all the time without much extra effort.)
(Killing blow = 1000mp5, and 1500 spirit for 15 seconds. If you proc Glyph of Shadow at this time, you get 30% of your spirit to become spell power => 500 spell power bonus for 15 seconds).
You always look for the killing blow so you can proc full mana regen on Spirit Tap in BGs.
This spec has extremely long and deep mana pool that keeps you with almost 100% uptime in BGs.
Even without killing blow, I constantly have 550mp5 due to the lesser Spirit Tap procs.
You get more spirit from Enlightenment talent, as well as spell haste. With this spec you achieve haste, spell power, crit and mana regen from a variety of talents, so that you have great stats even while you invested heavily into resilience with gear.
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Gameplay style: when your dots are up, and warrior is attacking you, spam mind flay. If you get kicked, cast a renew on yourself, refresh bubble if you can, and go back into Shadowform. If you are low, spam flash heal. You will have 100% cast protection because Martydom talent. and war kick is on cooldown. Go back to shadow form and spam mind flay again/get your dots back up.
In high pressure situations or competitive situation, use Power Infusion. You can use Power Infusion and spam Devouring Plague, never getting kicked, and doing high damage against class like warrior and ruining their day. You can spam Mind Flay for a large burst of damage. You can also use Power Infusion for other purposes, like to heal, dispel clean opponents.
Won't I die without Dispersion? No. In this spec you have massively high damage reduction from talents and gear. You will notice a massive difference.
The spec is built to be incredibly survivable, getting two talents that reduce damage by 12% and 15%, and adding onto that incredible resistance with more tools like Improved Power Word: Shield, improved Power Word: Fortitude stamina buff, Renewed Hope.
This spec benefits from crit and spirit.
In total, you get 64%+ damage reduction when enemies are attacking you (on 1600 resillience) You also get a buff that increases healing you receive, so that when you need to recover, you can do that more easily. All of this damage reduction is immune to armor penetration. No matter how much ARP someone has, you have 64% damage reduction. Currently I play with 30k hp. My power word shield absorbs 6k. You have extremely high effective HP because resilience makes your hp even bigger.
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GLYPHS
1. Glyph of Shadow
2. Power Word: Shield
3: Inner Fire
The goal of this spec is to increase your tankiness. Talents that You take talents that reduce damage and increase self healing. That is bubble glyph, inner fire glyph. That is talents into stamina buff, talents into Renewed Hope, talents into Focused Power, talents into Focused Will.
When it is time to heal, you have 3x Focused Will activated so your heals are 15% more effective, and you have a Martyrdom activated, so your casting is not knocked back at all. You have 12% damage reduction so you are tanky when you try to offheal.
Martyrdom allows you to cast heals without knockbacks. This talent allows for a flexible transition between tanky shadow and recovery off-healing. The strategy is to channel Mind Flay, and if they kick, cast heals. Enemy kick is on cooldown, and you have 100% protection so you can spam Flash Heal in their face if you need to.
Power Infusion gives you strategies versus classes that destroy you, like warrior. Just use Power Infusion, and spam Devouring Plague. You can power infusion and clean a paladin or priest in an extremely aggressive way.
Mind Flay does immediate damage, turns you towards enemies so they can never be behind you, slows enemies, and channels through walls, and activates Spirit Tap.
Draenei is a good race for this spec, as the racial spell heal is another cooldown that combos well with your high tankiness and playstyle to stand in shadowform.
I've been testing a hybrid spec for pvp, similar to yours, that uses the disc tree up until 4/5 borrowed time, then all remaining points into shadow tree. That gives you all the goodies of the disc spec (Focused Will, Power Infusion, empowered shields, etc) while also beeing able to use Vampiric Embrace and Silence. You also get reduced cd on Psychic Scream, (down to 26 secs with pvp set) and Shadowfiend (5 min to 2 mins). As for stats, you mostly focus on crit, since that gives you an absorb every time you heals crit. Stack up resilience on top of it and you become a though nut to crack on bgs.
Since nothing is free, so you trade some healingness by not picking Penance. However, you can still heal quite well with Flash Heal, which by using 2 pieces from T10 set that gives it a chance to do aoe healing, along with your other healing spells (Renew, Prayer of Mending and Holy Nova). As for glyphs, i use PW Shield, Renew/Inner Fire and Shackle Undead for those damn Dk pets.
Of course you be healing as much as a regular disc priest, but thats not the point. Your role is to be into the fray, dispelling, silencing casters, casting hots and aoe fearing as much as you can. Obviously you wont be soloing the whole enemy team, but also wont be always running or hiding in the corners as you normally would.
So it's more like a tank priest? I'll be trying this soon, but I can already imagine myself just slowed & rooted 24/7 because of no dispersion & no fade.
edit: I tried this in various 1v1s. Yes it is very tanky. Health bar stayed full most of the time. Drawback is it isn't as explosive as shadow so for example, fighting a good bis prot paladin is even easier (lol who knew) but takes much longer. And yes mana also took much longer to go down, but I tried a 1v2 against some warlock & paladin and it took soooo long my mana was at like 2k by the end of it, while as shadow it would have probably still been at 50% by the time they're both dead. Also, the amount of times plague & pain got dispelled without a worry was insane, while vampiric can crit 7k-14k if dispelled. It's probably best in BGs as a very nice supporter, and world pvp but only as self defense instead of a killing machine.
The truth is that it is not a big problem to lose Fade in a 1v1. Enemies get on top of you easily regardless. Priest cannot kite any class in a 1v1. Fade is more useful for BGs, Arenas, to LOS. Another thing: Dispel Magic removes a large amount of slows in the game. Reducing the value of Fade. Yes, this spec is designed for 1v1s / chill bgs. I designed it to strengthen the priest against classes that destroy priest: DK, warrior, rog. This spec makes the priest stronger against these class