Survive endless waves inside the arena. After every wave you choose:

Bank — leave the arena and move everything you found into your stash, or Continue — face a harder wave with better loot.

You only see the rarity of the loot you carry: what it is shows once you bank.

If you die, all unbanked spoils are lost. Every fifth wave, a boss comes.

Play together (in the lobby) opens a room for a friend. Each keeps their own spoils; a fallen hero can be raised by a teammate before bleeding out.