Qalena is right. Cross-Server Boss unlocks at level 54 (at the same time as pets), and if your room kills the boss, then the auction will appear. You need to do over 0.1% of the room’s damage to participate in the auction. In the auction, you use amethyst to bid for items such as pet cards, pet skin shards, pet equipment, and pet charms. Whoever bids highest gets the item. If someone else gets an item, you get a quantity of amethyst that depends upon how much damage (percentage-wise) you did to the boss, as well as on how much was bid to purchase the item. It can be slow to get amethyst at first, but when you do the rewards can be useful. It is also one of the easiest ways to pick up the various pets.