Acid sheath and death armor effects are very separate from what you're talking about I think.
First thing to look for is any spell animation associated with it, or a message that they've cast a spell, are surrounded by an aura, things like this. Aura's that can do this sort of thing typically go up right as they notice you. Next, look to see when exactly people are taking damage and what they're doing. Next, look for any saving throw challenge messages as you hit them. If there are saving throws going, this would explain why you're fighters aren't taking damage or don't seem to be with every hit (i.e. it could be a fort challenge OnHit or else take negative energy damage).
I'm still guessing this is an OnHit poison. OnHits are placed on skin or armor properties for effects like you're describing (or placed on weapons for the opposite direction effect). They aren't spells. If you're super curious, go into your toolset and create a row of monsters. Put OnHit Poison on their skin and put a different poison on each. There are a lot of different types. I'm guessing you'll find one that does negative energy damage with a save associated which would explain the inconsistent damage. It may be another OnHit property... this one just sounds funky and therefore I figure it's one of the poisons.
I don't really know of anything else that can be set up like you're describing. Dunno.
