if you finish all 10 waves, you (and you team) will win arena badges, you can exchange this to potions, and you may be lucky and win a surprise cube, that might contain a pit demon (it is rare).
also note that if you die you will lose nothing, exp, skills, equipment or blessing.
actually it gives you free mana and health refill and a temple teleport.
you can see more at Eric's videos
https://www.youtube.com/watch?v=fbpB-JXbx1Q
and
https://www.youtube.com/watch?v=qIDdO_SZPSA