Complete all of Swomp's missions. They are:
Light a buncha stuff on fire (30) - light 30 things on fire
Run over a car with your car - using a car, run over another car
Eat a sandwich when you're not even hungry - eat a sandwich when your health is full
Go on a successful date - dont get hit during the Winkydink mini-game
Crash the pizza party in Winkydink - secret, randomly apprearing room in Winkydink it has a checkmark in the minimap
Fill your inventory with slime (3) - after killing a slime, grab the slime ball they sometimes drop, repeat 2 more times
Start a polymerous relationship - have a successful date, then another one without your first date dying
Buy a dank gaming rig from the Styxcoin café