Mysterious Tablet

It appears at the dungeon entrance on its own once Golem is dead. Nothing summons it. Four cultists kneel around it, two devotees and two archers, and they stay passive until something makes them angry. Kill all four and the Lunatic Cultist arrives, shatters the tablet, and the boss fight starts on the spot.

The tablet itself cannot be damaged, talked to, or interacted with in any way. It is a prop with a trigger attached.

Chlorophyte armor's leaf crystal picks its own targets and it will happily shoot the tablet, which wakes the cultists before you are standing where you meant to stand. Anything that fires without being told can do the same: sentries, minions, a homing weapon aimed nearby. Swap the armor or hold well back until you are ready, because you cannot drag the cultists somewhere better and the fight begins wherever they die.

The tablet also blocks the Celestial Sigil while any player can see it, so a straight rush at the Moon Lord has to wait for the ritual to resolve. And despite the glow it gives off, it does not count as a light source for housing.

Run away during the Cultist fight, die to it, destroy all four pillars, or simply reload the world, and the tablet and its four cultists come back exactly as they were. Waiting it out takes half a day of in-game time. Reloading takes as long as reloading takes, which is why that is the usual answer when you want another attempt now. The exact spawn position shifts slightly if you change the blocks directly above and below it, which is occasionally useful if the default placement is awkward for your arena.