Not to be confused with the pre-Hardmode enemy, the Demon.
The Red Devil is a Hardmodeenemy that spawns in The Underworld after any mechanical boss has been defeated. While flying toward the player in a similar manner to that of the Demon, it fires five purple trident projectiles that deal extremely large amounts of damage upon contact. It is considered a more difficult version of the Demon, having more health and dealing much more damage. Similar to the Demon, it drops a magic weapon resembling its means of attack, the Unholy Trident.
While the contact damage of a Red Devil is comparably low, its trident attack is capable of dealing large amounts of damage even to players wearing endgame armor. The tridents are fired in the same burst-like pattern that Demons fire scythes in, although they are slower and move at a constant velocity.
Notes
(Old-gen console version) Red Devils will spawn prior to Hardmode if a mechanical boss is defeated prior to the Wall of Flesh.
Due to their relatively low knockback resistance, they can be easily stun-locked with rapid-fire weapons, such as Megashark or Chain Gun. However, this method still allows them to fire tridents.
Their projectiles are extremely powerful in Master Mode(Desktop, Console and Mobile versions), often being able to kill the player in one or two hits even with high defense.
If close enough, it is recommended to take the contact damage from the Red Devil in order to use the invulnerability to dodge the tridents fired, especially when playing as the melee class.
Trivia
The Bestiary entry for the Red Devil: "Conjuring potent spears of dark magic, these elite demons are a serious threat to any who dare enter their abode."