The Celestial Sigil is a boss-summoning item in Terraria, specifically used to summon the Moon Lord, a post-Hardmode boss. It's crafted from 12 / 20 of each of the Lunar Fragments. Unlike other summoning items, the Moon Lord doesn't appear immediately after the Celestial Sigil is used. Instead, it takes 12 / 60 seconds for the Moon Lord to actually show up. The Celestial Sigil can only be used under certain conditions: the world must be in Hardmode, the Golem must have been defeated, and there should be no ongoing invasions.