Ruthless Mode is an option in the Revelation Boss Hall unlocked by holding the R key for 10 seconds while the Boss Hall is open. When the player selects an unlocked boss, they normally have either an option to fight a regular variant of said boss or a champion variant. However, for some bosses a third option can be selected termed "Ruthless" which imbue's the boss with much harder attacks. Currently only 3 bosses have a Ruthless mode, that being Punker, Aragnid and Maxwell.
When Ruthless mode is selected, the boss normally appears with beaming red eyes and a dark blue pallet.