Twice now I have encountered something odd, so I thought I would ask about it. When battling a Death magic wizard, they constantly cast drain life, and it does almost nothing. Then I fight the same wizard in the city with their wizard tower and drain life ONE-SHOTS my unit stacks. One-shots ultra-elite dwarf hammerhands stacks, 24 hits!

Is this a bug? Or some mechanic of Death magic that I do not understand?

I considered the possibility of resistance modifications, for instance a praymaster or high prayer on my side or black prayer on their side, but neither has been present when the anomalies occurred.

Wow that is extremely buggy. Are you playing normal version or Insecticide?

With the base 9 Resistance that Hammerhands have, a fully pumped up Drain Life spell only reduces their resistance by -8 and so they should at max only take 9 Damage in total at most I believe.

For more info on how the spell works you can check out the link below.

masterofmagic.wikia.com/wiki/Life_Drain

Sadly then it may only be a bug with that version because I have never had it wipe out someone with more than 10 hp.
I am not sure where to report Insecticide bugs though that will be seen by the developers of it, or those who could fix it.

Another thing that is totally off topic but I know you follow the WoM thread. I kind of like games that give spells that are useful all through the game and actually "power up" the more you use them like in Secret of Mana (RPG for SNES). Offhand I cannot think of a TBS game that does the same but I think I would like it.