That's definitely an interesting idea, but if the wizard is that powerful, how did the Monk/Cleric end up killling him?
That death curse would be ideal for a mad cleric. He'd be able to exist and wreak havoc on an unsuspecting world. No matter how horrible the wizard, there is no way they would ever want their killer to be alive forever.
Also, does he age? Does it cap off somewhere or is he still spry and young. You're giving the cleric the ability to take out anyone he believes is bad, in his skewed brainpan. He could've been the catalyst in the Salem witch trials. No wizard would want something like that for their kind.
Maybe a better curse would be to trap him in the veil between reality and the nevernever. Unable to cross into either, he is forced to observe each side.
How does he kill him? In books, we learn that wizards can die the same way other people can, it's just that their magic affords them more options to rescue themselves.
The cleric could have used poison (slipping it the wizard's drink, stabbing him with a dagger coated with it, or perhaps in a gaseous form). Perhaps, he had minions who wore the wizard down before he slipped in to deliver the final strike. Or perhaps an old favorite, he captured the wizard while asleep and had him burned at the stake. It's very possible that a mundane individual can kill a wizard. The cleric is crazy not stupid.
As for aging, that's also one of my questions. Would a curse keep him young ? If you go with the true idea of a curse, it would not keep him young, it would keep him alive with all the problems of aging. It also has to be considered how passive or active a curse could be. A curse that would require learning all the knowledge of mankind would demand constant research of
EVERYTHING! Go to an encylopedia and open to the science section and see how many different branches there are and then do the same with arts, etc. To this mix, add an addiction that makes coccaine seem like a sweet tooth, this curse becomes very sinister and tragic . Not to mention, A cleric who is forced to live beyond his normal span is denied entrance to Heaven. (Although, the Monk is denied this due to his actions that started the curse in the first place.)
As for his place in a game, the Monk would either be an NPC who could be questioned for esoteric answers or a truly formidable villain with the knowledge of ages to use against the heroes.