Defeat Splat by purchasing his challenge card and challenging him in the Brawler's Guild.
You may already be familiar with Blat, one of our Rank 2 challengers here in the Brawler's Guild. The challenge card you've earned belongs to Splat, its older, nastier sister.
Try not to get yourself killed, <name>. And if you do, at least make it entertaining for the rest of us!