it is a penalty, not a spawn
You will not find one of these anywhere in the world and no item calls it. It only appears during the Lunatic Cultist fight, after Golem is dead, and only because of one specific mistake: attacking a duplicate of the boss while a Phantasm Dragon is still in the air. Learn to pick the real cultist out of his copies and you can finish the fight without ever meeting one.
if one is already out
It charges flat across the screen, sweeps past you, turns and comes straight back, trailing afterimages the whole way. The pattern is close to Mothron's and the answer is the same: move across the charge instead of trying to outrun it down the same line. What kills people is dodging this at the same time as the boss, so either burn it down immediately or reposition until its lane matches something you were already avoiding. Its head is a fairly direct nod to Cthulhu, and to the Moon Lord you are about to summon.