The Dark Auctions is a secret location that puts up items for bidding.
A Dark Auction waiting for atleast 2 players to join
Every hour or so, a Dark Auction event will prompt everyone online to join. This is an event that allows players to bid on special items, not so easily obtained anywhere else.
You may also start these events by yourself, using a Sirius Dust summon item, or a Sirius Charge which allows you to reset your cooldown only. These items are won from Mythical voting crates or in the Dark Auction.