You enter the arena. You will fight 5 monsters/NPC's, one at a time. After defeating all 5, you will face Magni the Bison. After defeating Magni you can redo this quest. Tactics on how to beat the quest are given at Guide to defeating Magni the Bison. If all else fails, learn You Move Like a Dwarf and keep Magni the Bison crippled with skills and/or Slow him down with hexes. Eventually if you stay alive long enough, the match will end; and you will gain a Decisive Victory.