This was an interesting QA, but there are a few issues.
When I entered, there were/was an injured SPOILER(s) standing near where I spawned. Needless to say combat began very quickly, but I think this bug is perhaps a fluke from another player having left/died there.
The second is that I could find no way to close the quest. After I defeated the SPOILER(s), I pulled the chain to exit. Although my journal instructed me to inform the arena master of my victory, I only got very general, "Wow, how impressive," messages. I am not sure how to end the quest to recieve end gold and XP.
Nothing spawns when you enter the arena, only option with the quest giver is to say you lost. I blame Semli.
Same happen for me. completed the quest, but no option to end it it left me expless and lootless. :(