Hmm... I agree with sessions, maybe you could just give the NPC Thana Fragments. If not, I suggest Donation Coupons (like the Renter NPC). And I don't like the idea of losing when you die, so I also agree with the badge thing - you (and your party, if you have one) will get a badge once you go inside the room, then when you die you can talk to the Thanatos NPC again and show the badge then you can go back in and start killing again. :D
I have an idea though. A lot of people would like to use the Thanatos NPC, and like what Xtopher said, players would just use different accounts so it wouldn't be fair, other players won't get a go. So what I'm suggesting is maybe there could be more than one room? Like it could be a big place, with lots of different rooms in it and the rooms could be numbered. Or something like that, just so other players can get a go at using the NPC and killing the thanas.