So both Hearstop Aura and Necrotic Aura is stacking for each nearby Death Knight, I am not sure if that is a bug or not. However if you have 7 Frost DKs on you, your cooldown recovery rate is at 8%, meaning it takes more than 10 times longer for your cooldowns to return. My Divine Shield which is usually 5 minutes, end up taking 1 hour to recover!
Sounds a bit OP in my opinion.
That is just Heartstop Aura. Necrotic Aura will also stack, and since you multiply them, the magic dmg increase ramps up real quickly.
If you want an example of how strong it will be with many Death Knights, check out an oldschool video when 40 Death Knights killed the faction leaders of the Horde https://www.youtube.com/watch?v=L8qQP
This is done during early BFA, now it would go even faster.
The total DMG increase in that clip, is about 2000% magic DMG boost!
This can be really really useful for Rated Battlegrounds. Or even Arenas. If you were to use double Frost in arena, people nearby would have half their normal recovery speed on Cooldowns.
In Rated BGs, if you stack 7 DKs, their Necrotic Aura will add 70% damage boost to magic sources. However a great strategy is to split them between the enemy healers, since that will mostly shut them down with REALLY long cooldowns.
Overall, it is a really silly but fun Interaction (even if it is a bit gamebreaking for the receiving people...)