- The Fighter gets a temporary boost of one point to his armor class.
- The Cleric regains a significant portion of his health, if injured.
- The Mage gets more Mana. If the player can have any more Blue Mana, it fills that first, and then gives the remainder in Green Mana.
The effect will also be applied to all nearby players in co-op, thus it makes sense to gather together when using the Incants to get the most of them. Perhaps because of this multiplayer nature of the item, Incants, at least in the original game's maps, do not appear in single player.
Ambit means a sphere of influence or scope of actions/thoughts/words. This fits well with the multiplayer effect. Latin root: ambitus (circuit). You could roughly translate the item as "magical sphere-of-influence spell".