Challenge Card: Ouroboros

Challenge Card: Ouroboros is a Challenge Card that summons Ouroboros to fight in the Bizmo's Brawlpub or  Brawl'gar Arena.

Source
The item can be bought for from:
 * Card Trader Leila in Bizmo's Brawlpub
 * Card Trader Ami in Brawl'gar Arena