Now, i could be wrong, but when I was playing as UB, I noticed that after I upgraded Post Mortem to Plague, the creeps I attacked stopped exploding. Assuming that I'm correct that choosing Plague wipes out the prior Post Mortem, is it worth upgrading? 300-450 damage over 30 seconds lategame is almost nothing, and won't wipe a wave of creeps on it's own. Post Mortem, on the other hand, seems to one-hit creeps caught in the explosion, so UB can wipe a wave of creeps just by killing 1-2 in the right place.