Arach, the Vile Spider was a boss that has since been removed from the game.
Arach's boss room is a simple rectangular room with some platforms for the player to stand on.
Arach will walk somewhere random on the platform he's currently standing on, with the intent of dealing contact damage with the player.
Arach will jump onto a wall, or back onto the floor if he's already standing on a wall.
Arach is weak to Mortar. Hitting Arach with Mortar does nothing special other than the double damage, however.