Ancient Doom

Nothing you do summons this. The Lunatic Cultist starts placing them once you have taken him below half health, and only in Expert or Master. In Classic the attack does not exist at all, so the first time most people meet one is on a difficulty step up.

Each one hangs motionless in the air and only hurts you if you run into it, which is the trap. It looks like scenery while you are busy dodging the boss. After a few seconds it bursts into four beams that travel straight up, down, left and right, and those beams cannot be shot out of the air once they exist. In Master Mode a single one can end an attempt outright.

Break every Doom before its timer runs out. If a group went up at once and you cannot clear them all, get off the horizontal and vertical lines they are sitting on. Standing diagonally from everything on screen is the safe answer, which is a good argument for a wide flat arena with room to move at an angle instead of a tight box.