Selenian

The Selenian is a Hardmode, post-Golem enemy that is spawned by the Solar Pillar during the Lunar Events. It is a ground based enemy that wields two flaming swords. It can curl up and fly towards the player at high speed. When it does so, certain projectiles you have fired at it will be reflected. This is very dangerous as some weapons like the Sniper Rifle and Meowmere can potentially deal over 300 damage to the player.

Among the things they reflect:
 * All bullets
 * All arrows
 * All darts
 * Some sword beams, such as those from Meowmere and Christmas Tree Sword, but not all forms. (listed below)
 * Scourge of the Corruptor projectiles (main)
 * Nebula Blaze projectiles
 * Frost Hydra, Ballista, and Flameburst projectiles
 * Lunar Flare projectiles
 * The Horseman's Blade projectiles
 * Most special ranged weapon attacks, such as the phoenix from the Phantom Phoenix or the special rocket from the Vortex Beater
 * Paladin's Hammer projectiles
 * Nebula Arcanum projectiles

Among the things they do NOT reflect:
 * All rockets (except Stynger, Jack 'O Lantern Launcher and Electrosphere Launcher)
 * Terra Blade's projectiles
 * Influx Waver's projectiles
 * Flairon's projectiles
 * Terrarian's projectiles
 * Star Wrath's projectiles
 * Last Prism's beam
 * Lunar Portal Staff's beam

Trivia

 * The word selenian means "lunar" and refers to Selene, the Roman goddess of the moon and sister of the sun-god Helios.