Its dmg is not split by the amount of mobs hit, it will do its full dmg to every single mob in range, on top of that on a Blood DK who has the highest HP pool of any tank and stacks Vers it really really REALLY starts doing absurd dmg.
Tanks are running 80 corruption in M+ some abit higher so they take 5-8% more dmg and reduced healing by 5-8%, nd they will be doing competitive if not higher dmg on trash packs if they get lucky procs.
the thing is they can shrug off the dmg from the corruption affects and have Null barrier Minor so its just exploding magic dmg all run.