With the Gods short of one fighter and needing a replacement it was decided Hades would be a good candidate; for reasons unknown they decided you would be a good person to convince him. Not really in a position to refuse you begin the journey to Helheim and surprisingly meet Hades on the Bifrost making his way to the Valhalla Arena already.

If you were sent to convince me there is no need; I will take Buddha's place as the Gods representative.