you have to beat underworld HM per usual. but there is no chest. you have to go to the hall of judgement and fight dhuum. he summons bone minions which do no damage against SF (hehe), but are like warriors on crack. if you die once you go to forgotten vale and come back as a weird spirit with different skills. you have to protect the reapers as they put dhuum to rest and then kill dhuum. we were all on vent screaming at each other, and still don't completely understand what happened. but once he's done, there's a chest, with new drops. some of our guys got some sweeeeeeeet stuff (luckers!)
some other things: dhuum has a few crazy skills/spells. he has a 200 or so dmg scythe attack. a nasty hex. a summon minions spell. and a large aoe fire spell.
for those interested in how to beat uw, we used the mobway tactics. not the fastest, but we'd rather be safe than sorry.