The Scroll of Resistance is a quest reward given by Malah after completing the Prison of Ice.
It increases your base resistances by 10 per use. Since the quest can be completed three times, once for each difficulty, the maximum reward legitimately possible is +30% resist all.
Due to a bug, if the hero dies, this bonus will disappear until you join a new game[1]. The Character Screen does not show this bug. This is not a concern to Hardcore characters, as Hardcore characters are permanently gone once killed.