Nuclear Throne Together allows you to define additional custom crowns. This is done by creating a yourcrownname.crown.gml file in game's main or appdata directory and loading it via /loadcrown yourcrownname. The file can #define the following scripts to be called by the game:

init

Executed when mod is loaded. Load resources here.

game_start

Executed on game start. Reset global variables here.

step

Executed every step while the crown is active.

crown_name

Should return the crown' name.

crown_text

Should return the crown' description.

crown_tip

Can return a crown-specific loading screen tip.

crown_avail

Should return whether the crown should appear in the selection screen (insert loop check / other conditions here)

crown_button

Executed for crown' selection icon. Should set up it's sprite.

crown_take

Executed when the crown is taken.

crown_lose

Executed when the crown is lost/given up.

crown_object

Executed for crown "follower" object when it appears. Should set up it's sprites.