The Red Devil is a hardmode enemy that spawns in The Underworld after any one Mechanical Boss has been defeated. It is considered a harder version of the Demon, with more health and much more damage. Like Demons, they sometimes drop a Magic Weapon with the same mode of attack, namely the Unholy Trident.
While the Red Devil itself does not cause much damage on contact, its trident attack is capable of dealing large amounts of damage even to players in any type of armor. These tridents are fired in the same burst-like pattern that Demons fire scythes in, although they are slower and move at a constant velocity.
To defeat it, attack only while it charges his trident. While he shoots, focus on dodging only, as the Red Devil's hits are deadly.
In the Xbox 360 edition, If you defeat one of the Mechanical Bosses before the latest update, they won't spawn.
Notes
If you defeat any of the three mechanical hardmode bosses before hardmode is enabled (through use of the appropriate summon items), this will allow for the Red Devil to spawn at any time. If you don't want them to appear, avoid fighting the hardmode bosses pre-hardmode.
It is holding an Unholy Trident, as proven by the death message (Player) was (death type) by Unholy Trident.