Next spell with a cast time interrupts that school for 8 sec and causes 8288 to 8712 Shadow damage.
This is one of the abilities you might come across in Jaraxxus encounter on heroic mode.
The Mistress will have this extra ability.
What to do?
1. Cast a spell from a different school than what you are regularly casting
Example: If you are a holy priest start casting a Mind Blast (remember, must have a casting time).
Result: you will get locked on all your shadow spells, but you will still be able to heal.
2. Start casting a spell BUT DON’T FINISH IT – (this is what I do)
Exemple: Doesn’t matter if you are dps or healer. If you have the debuff just start casting ANY spell but be sure you don’t finish it. Like, a holy priest – start casting a Greater heal, but don’t finish it.
The debuff will go away, you won’t be locked so … happy time! you can continue and do your job.