[lineere]

Two possible issues;

1) Soultures can be captured. Some other players have stated that this might be a bug.

2) Drain dry can be used on NPC enemies that are at any HP, not just 0. This causes the enemies to die the next round. 

Hope this helps. I would post screenshot, but I'd have to make a video to show how it works. Either way, I think you can figure out what's going on from the info.


Edit: 
Tested on Alruane boss, it works on bosses too.

[Neus]

I believe this was corrected in a hotfix, if someone wants to verify that it doesn't work on live, please let me know!