The Celestial Sigil is an endgame-tier summoning item used to bypass defeating the Celestial Pillars and immediately summon the Moon Lord. It can only be used once Golem has been defeated in the world it is used in.
If the event has already been triggered by defeating the Lunatic Cultist, the sigil won't work, and will not be consumed. Similarly, if any other boss has been summoned it will not work.
The Sigil will also not summon the Moon Lord if any of the cultists are on screen.
When used, the game will announce "Impending doom approaches..." at the bottom left of the screen giving you around a minute until Moon Lord spawns.
Due to its recipe demanding all four fragments, this item can only be crafted after defeating the Celestial Event at least once.