You found the clown costume.

You found the baby costume.

You found the miner costume.

Cleared a level with just the throwing knives.

Cleared a level with just the flamethrower.

Killed all the school children.

Cleared all levels with B rank or better.

Cleared all levels with A rank or better.

Cleared all levels with S rank.

Unlocked all the achievements.

Completed the game.