Scurge: Hive

Unlock Hard difficulty level:
Finish the game on the normal difficulty level to access hard mode.

Unlock Insane difficulty level:
Finish the game on the hard difficulty level to access insane mode.

Unlock Ultra difficulty level:
Finish the game on the insane difficulty level to access ultra mode.

Unlock Boss Rush mode:
Finish the game on the hard difficulty level to access bosh rush mode.

Costume palette swaps:
Finish the game on the normal difficulty level.