| Asmondeus lieks caek |
Well he wants what ALL devils want SOULS! [Insert Dramatic music here]
You see The Hells are a strick heirarchy, Devils advance in the ranks by doing service to their superiors.
In most cases the easiest way to do this is by daming the souls of the living (or Coin as they call it) to Hell or simply capturing them and draging them down there. All devils (save Asmondeus) are servants to someone greater than them selves so this mighty fiend is most likely the b&$+@ of some lesser Duke or ever an ArchDuke, but since he is quite the badass none the less he will most likely want all the souls of an entire small to medium sized village or the souls of one or more powerfull sorcerers (Moar arcane energy to reap). Seing how it is an evil party that could be attained if the party was willing to take the risk.
Read Fiendish Codex II: Tyrants of the nine hell for more info on this.
EDIT: Whoops some pretty much already talked about this, sorry.