Mecha cultist

The steel cultist is a boss that you can fight if you kill all 3 of the mechanical bosses and the lunatic cultist all at once (and the moon lord twice) and once you enter the dungeon it'll be invaded by probes then after killing them the message "you'll regret that" will appear in chat and then when entering the hallow at night while on fire will summon the Mecha cultist (also will only spawn if your in expert mode and hardcore mode)