Skodt wrote: I went to the ambitious imp and finished his quest (8 tokens from dwarf fighters), and then, it was impossible to find him again on Depth 18.
Sometimes it's hard to spot the Ambitious Imp as it is almost completely transparent. If you have a Potion of Mind Vision and your willing to use it, use it to see if you can find it. If you see a 3x3 spot but don't see anything there, look closer and you might see it there. Make sure to check thoroughly if you are unable to find it.