Looking for fantasy novel that opens with a wizard performing a resurrection spell that doesn't go quite as he planned. It was either cut short or didn't work and he ends up botching the incantation, creating a kind of blank slate of a man, rather than this bad-ass old battle mage of legend named Balthazar.
I think the wizard dies shortly after, sending this blank kid to the nearby kingdom. The kid hooks up with the royals eventually. Learns to ride a horse and wear armor and all that good medieval stuff. Then starts to have vague memories of his "former" life.
All the while, the kingdom is under imminent threat from a neighboring army - an army that hopelessly outmatches the kingdom's. The kid was resurrected to help defeat the army, which is kind of a lot of pressure, since he's still learning to tie his shoes and not stab himself in the eye with his fork while eating.
The book ends with the kid riding into battle and all the the memories come flooding back to him and he realizes that he is the once and terrible Balthazar. And I'm pretty sure he's like "BALTHAZAR!!" as he rides into battle and pretty much lays waste to the other army.
I read the book around 2001 (though it could have been published much earlier) in Australia (though I suspect the author was English). I think the cover was green (shrug), probably with a wizard on it or something (double shrug).