ToME: the Tales of Maj'Eyal

I may be wrong, but aren't negative resistances meant to increase the damage taken by a given source?
I mean, 100 base fire damage on a -50% fire resistant creature should deal 150 damage. I tested casting flame on trolls, they take the same base damage as 0% fire resistant creatures.

Yeah resistances are bound to 0/100 currently, I was pondering making it -100/100

Well, for what it's worth, here's my opinion:

I see at least 3 points that would favor implementing negative resistances:

1) It's strong tradition in roleplay games that specific monsters are more susceptible of taking damage from a given source: trolls from acid and fire, undeads (expecially skeletons) from fire, fire elementals from water, and so on. I think any person with a decent background in rpgs, approaching ToME for the first time, would almost be expecting this.

2) Having the opportunity to take advantage of known weak points can lead the player to develop specific strategies to face tough enemies, thus learning skills and spells otherwise neglected. This adds value and variety to the game experience, opening new ways to experiment with character builds and carefully plan its development, even in prevision of later stages of the game.

3) Having some artifacts give negative resistances to the character itself can help mitigate the effects of artifacts that otherwise would be too powerful; this would force the player to switch to the equipment that best suits to the current situation, instead of wearing the same killer gear all the time.

Does this mean the -20% fire resistance on the Shield from the Old Forest actually has no effect, unless you are also wearing a source of (positive) fire resistance?

I think it has effect, but it can decrease character's resistance up to a minimum of 0%.