You use chants at Incantation Shrines in order to temporarily boost party stats. They can be QUITE useful in dangerous situations. Here is where I've found chants in the game. Note that you don't have to memorize the words or anything - you simply pick a chant from your in-game list.

Lesser Chant of Fortification - Act 1, Ch 2 right as you enter the jungle, from Esselte
Lesser Chant of Dexterity - Act 1, Ch 2 near same spot as above, in tower
Lesser Chant of Strength - Act 1, Ch 3 just after first tower you burn
Lesser Chant of Intelligence - Act 1, Ch 3 in same spot as above
Mysterious Chant - Act 1, Ch 3 behind waterfall after Tower 2 (gives random item)


Dungeon Siege 2 Walkthrough