The Shadowflame Apparition is an enemy summoned by the Goblin Summonerenemy during a HardmodeGoblin Invasion. Summoned in swarms as projectiles, they pass through blocks while attempting to ram into the player. They are immune to knockback, and both they and their caster move very quickly, allowing them to easily overwhelm the player.
An apparition is a name for a supernatural phenomenon, such as a ghost or phantom. Terraria incorrectly spells it as apparation.
The Bestiary entry for the Shadowflame Apparition: "The shadowflame magicks are powerful spiritual remnants of twisted goblin ancestors, wrought from powerful summoning."