Moon Lord can be summoned with a Celestial Sigil before defeating the Lunatic Cultist in any world where Golem has been defeated. If the player leaves the world after the message "Impending doom approaches...", Moon Lord will not spawn.

What is the item to summon Moon Lord?

The Celestial Sigil is a boss-summoning item crafted out of 20 of each of the Lunar Fragments. It is used to summon the Moon Lord without going through the Lunar Events.

What happens if the moon Lord kills you?

After that, he will explode one final time and the only things left behind are his skull and ribcage that disappears a few seconds later. Moon Lord dying in the jungle. If all players are dead when this happens, he will disappear and drop no loot.