Milkyway Weaver

It belongs to the Stardust Pillar, so you only meet it once the Lunatic Cultist is dead and the lunar event is running, and only in the stretch of sky that pillar occupies. Nothing summons one directly. They come out of the pillar's own spawn pool, which also means killing them counts toward stripping the shield off it.

Treat it as a floating worm with one weak point at the front. Gravity does not apply to it, so it loops in from angles a ground worm never could, and unlike the Crawltipede over at the solar pillar it will come for you whether you are flying or standing on the arena floor. Dragging a piercing weapon down its body accomplishes nothing. Every hit has to land on the head. Its segments hang loosely instead of locking together, so the faster it moves the further it stretches out, and a quick one can be strung across most of the screen with the part you need to hit somewhere off at the end of it.

Chasing you is not what makes it dangerous. After a few seconds on your tail it slows, sits, and charges up while a blue glimmer builds and gets brighter, and at the end of that it drops a fresh Stardust enemy into the fight in the same constellation pattern the pillar itself uses. What comes out depends on what is already alive: a Star Cell, a Flow Invader, or another Weaver, and the whole thing fails outright if none of them are eligible. Then it goes straight back to chasing and runs the cycle again.

That makes every one you ignore a slow drip of reinforcements around the pillar, so clear these before you turn on anything bigger. The glow is a long and obvious tell and it is nearly stationary while it happens, which is the cleanest window you will get to close in and burst the head down before the summon lands.