Bloodied Scroll

The Bloodied Scroll (previously Blood-Soaked Invitation) was one of two ways to gain access to the Brawler's Guild. The other way is the, mailed to members of the guild who have reached rank 7.

Note that once used, the account is able to join the Brawler's Guild, not just the individual character. Don't waste time farming the rares or gold on the Black Market Auction House on alts!

Source
Invitations have a rare (roughly 5%) chance of dropping from any of the Pandarian champions, Zandalari Warbringers or faction NPCs for I'm In Your Base, Killing Your Dudes, and a 100% chance of dropping from Willy Wilder, an NPC often found near the Black Market.

Invitations are also sometimes sold by on the Black Market Auction House, with a  starting bid and a 20-hour duration.

Achievements

 * The First Rule of Brawler's Guild (Season 2)