This spell seems like a waste, I can only think of one good use for it, and that would be PLing someone. If you can remove 5200hp from a mob without doing any damage to it, that will make it a lot easier for them to kill and get the exp.
I tried it out a little in FOB, and it does nothing at all; a skeleton that dies to my 49 point DS still dies to it with or without getting the debuff. It does however aggro him. If I hit the mob, its body stays around so I can loot it, but if it dies to DS, with or without this debuff on, it fades... so my conclusion is that whatever this spell is doing, it doesn't count as damage.
That said, I'm still at a loss as to how this is the least bit important for normal soloing, grouping, or raiding. Mobs I'm killing have so many HP that 5200 is a drop in the bucket, and since it only reduces max HP, there would only be an effect at the very start of a fight.
Meanwhile, I don't get any benefit from my damage focus, from spell damage mod, from various crit hit and spell damage AA. Its just gonna be a flat debuff to max HP, and even that is going to fade, so only if I can then kill it before the first tic does even that count.
If anyone has any ideas for how to make this useful even on rare occaisions please speak up :)