Can you summon Moon Lord again if you killed him already?

I didn't kill Moon Lord when I summoned him with the pillars! Can I summon him without the 20 pillars, or in some other way?

Solution 1:

Yes you can summon him again. When you defeated the pillars they would have dropped celestial fragments, using 20 of each at an Ancient Manipulator (dropped by the Lunatic Cultist) will let you craft a Celestial Sigil. This can be used to summon the Moonlord.

However, I’d actually recommend using the fragments gained from your first attempt to craft weapons for you class of choice, as they’ll likely be better than what you have currently. Then fight the Lunatic Cultist and defeat the pillars again.