First of all, KT is not a complex fight for us healers. The only difficulty is mana wise because it is way overtuned here on feenix, and thus very long fight. So it goes without saying that we'll have to be very try hard consumables wise, and prepare a lot of mana pots (one each 2min - 30 min fight).
During P2, KT uses a spell called "Frost Blast". This spell cast an iceblock on a player, dealing 130% of his HP pool in a 5s dot. Each player too close to the targeted iceblock will receive the same debuff.
For this particular spell, I feel that 3 addons functions should be mandatory.
The only two raid frames allowing working and accurate range check / debuff check are Luna and Sraidframes.
Please, do not rely on Grid to heal Frost Blast. The debuff check bugs out half the time with its vanilla version.
Everyone is in the center of the room.
3 type of adds will atatck us. Melees are on the big guys and casters on the banshies.
This means noone is on the skeletons (depending on caster dps ofc), so that's why healers have to kill them.
There is very few healing to be done duriong this phase, and it's very important that skeletons die. So this is a very important task for us healer to actually focus on. And not only taking it as "I'll just throw a smite here and there and wait P2".
Only a few healers should be assigned to the actual healing, while the others dps the skeletons.
If a skeleton do reach the raid, it will explode and deal damage to everyone. In that case, each priest should throw a PoH to top up its group quickly.
Basicly, healers are spread around the circle where KT is tanked (this way, everyone has a healer in range if he gets frost blast debuff).
Each group stay close to its healer so they are in range for PoH (important spell on this fight).
Healers are closer to the center than casters, so they have all melees in range.
Everyone is spread out enough so there is no iceblock chaining.
This is a verylong fight, don't waste mana with flash heal (other than on frost blast)
Then there are a few mechanics to be aware of as priest :
Void zone : The visual is a big void zone under a player. If you're on it, move so you don't die. The actual effect is bigger than the visual effect, so do'nt stay on the edge neither. When you move, don't chain the other people. If you see someone moving out of its void zone, make some space for him so he doesn't chain you.
Frostball multiple Everyone gets damage in the raid. PoH your group. Don't waste mana with flash heal. Don't forget to top tank up after this spell. Unless there is a risk for Frost blast, the raid don't need to be insta topped, unlike the tank.
Frost blast If there are more than one frost blasted guy around you, shield the first one and flash heal the second one. If there is only one you can heal, GH him.
Once again, this is the most critical problem as healer. Do NOT let anyone die from that. Start casting as soon as you see the debuff, and do not wait for the HP target to drop.
Sometimes, melees will link each other, resulting in a lot of frost blasted people at the same time, including the tanks.
In that case, each healer should be assigned on one melee so we don't all focus on only one guy. (not so usefull as horde considering sham chain heals)
Lastly, I used to pre shield myself before each frostblast. In case healers around me slacked or were iceblocked themsleves.
Mana detonate It burns 50% of your current mana pool, and chains to close targets.
Mind Control If after your MC you're far away from your original position, watch out for frost blast chaining. If there is a risk of frost blast soon, and that you cannot reach your spot safely, wait in a safe zone untill you see next frostblast happen. If there is no safe zone, chain ONE guy, instead of multiple (stack on said guy, preferably not mana user).
Scarabs will spawn, only 3 can be shackled, otherwhise KT will dispell the others.
So 3 priests should be assign to that + backup if they died earlier in the fight.
Anyone dying during thsi phase will buff the scarabs makign tank healing very difficult.
This is a specc for KT and KT only.
Unlike on other fights, renew is not a spell that will be very usefull on KT.
Meanwhile, we take a lot of damage (thus spell warding) from forstballs and frostblast, and PoH is often used during multiple frostballs.
Concerning spiritual healing, this talent is apparently broken on feenix and renew is the the spells that benefits the most from it. Thus the choice of improved PoH and mental strenght instead. This is a very long fight, costing a lot of mana and PoH (costs tons of mana) is highly used due to multiple frostballs.