To summon the Moon Lord, you have to complete the Lunar Invasion. To summon the Lunar Invasion you have to defeat the Lunatic Cultist. Every single time. This can get quite time consuming and frustrating, when trying to fight the Moon Lord multiple times. My solution: the 'Luminous Moon Rock'.
Upon defeating the last celestial pillar during the Lunar Invasion, 1-3 Moon Rocks will drop. You'll need 1 Solar, Stardust, Nebula, and Vortex Fragment along with a Moon Rock, to create a Luminous Moon Rock, which spawns the Moon Lord. If you're lucky, you'll get 2 extra boss fights out of it, or if your unlucky, you'll only get 1.
Upon defeating the last celestial pillar during the Lunar Invasion, 1-3 Moon Rocks will drop. You'll need 1 Solar, Stardust, Nebula, and Vortex Fragment along with a Moon Rock, to create a Luminous Moon Rock, which spawns the Moon Lord. If you're lucky, you'll get 2 extra boss fights out of it, or if your unlucky, you'll only get 1.