Wow, that was a quick and thorough suggestion Raze! Thanks.
I already tried a previous save so it's all ok now(one where I didn't destroy the Red Hammer Tribe huts & catapults so maybe that had some effect), but I'll still give some more info:
- I'm playing the steam version (so it's patched up), and I downloaded Larian's DRM removal fix
- I'm playing no hard difficulty, though when I died the first time I switched to easiest, but still had no effect
- creature summoned and obeying my commands
- I did think of using a potion

and it was one of those 384 health regenerated (extreme or ultimate, can't remember the name)
In any case, thanks again to both of you
