Will I get another shot at unique monsters if I leave the current level?

While exploring on L8 (400' down), I met a certain serpent-tongued agent of a well-known wizard. Between his insults I was doing pretty well against him, until while chasing him down a hallway I fell through a trap door to the level below.

Obviously, I can't go back to the same dungeon level I was on. (Well, I can go back to the same depth, of course, but it's a whole new map.) But will I get another shot at finding and slaying this worm, or do unique characters like this spawn only once in a given game and then that's it? I'd really hate to let his slights about my mother stand, but I'd like to know if I'm just wasting my time hanging around or if I can push on deeper into the dungeon.