Ancient Light

This does not spawn in the world and nothing summons it. The Lunatic Cultist begins firing them once his health drops past the halfway mark, in fans of five that spread apart as they travel. They are counted as enemies rather than as projectiles, which is the reason they can be shot down at all.

A weapon that covers area clears a whole fan before it reaches you, and that is almost always a better use of the moment than threading between five of them. The complication is that later versions gave the lights a real pool of health instead of the single point they used to carry. Homing weapons and minions now retarget onto them and dump their damage there while the cultist keeps casting untouched. If your build picks its own targets, expect the back half of the fight to crawl, and bring something you aim by hand as a backup.