Book identification - quest to defeat impending magical death

I read a book (or trilogy?) in the 90s about a boy who gets cut on the chest by a knife. It's a magical wound that will kill him within a few days or weeks. A good wizard assembles a group of guardian-type people to take the boy on a quest to defeat the evil ruler who marked him. One in the group is a girl with some kind of mind powers who makes the boy forget about his impending death. Another is a warrior with black eyes named Scythe after the sword he uses. The boy turns out to have magical powers as well and is eventually revealed as an elemental (?) with power over the elements.