Eldric is an astronomer specializing in celestial phenomena and omens. He noticed the Eye of Cthulhu stirring in the night skies and wants the player’s help to prevent impending danger.
Eldric has robes adorned with star patterns, carrying a staff topped with a moon crystal.
Eldric can be found in the unbuilt Observatory in Ravencrest after the King Slime has been defeated.
Eldric notices the player's involvement with defeating King Slime and offers to share his concerns about the Eye of Cthulhu. This quest is only available after the King Slime questline is completed.
Starting: Speak to Eldric in Ravencrest
Eldric greets you, and notes his unbuilt observatory. Then, however, Eldric mentions that something's messing with the stars - a great eye.
Eldric sends the player to collect 5 Lunar Shards, rare fragments found on wandering enemies at night, symbolizing the growing power of the Eye. Additionally, he asks you to get a bizarre concoction that Garrick carries called the Lunar Liquid. From these two, he can make a potion that'll send you right to the Eye's hideaway. He'll be building his observatory while you work.
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Next: Talk to Garrick for the Lunar Liquid
Garrick hands you a “Lunar Liquid". He says that he had been tasked with keeping it for just this situation…and that Eldric seems to undersell the Eye's strength.
Next: Return to Eldric with 5 Lunar Shards
Eldric thanks you for the shards.
If you haven't yet talked to Garrick and obtained the Lunar Liquid, he'll tell you to talk to Garrick to move along. After doing so, returning to Eldric will continue the quest.
If you have already talked to Garrick, he'll give you the Lunar Concoction, a potion that, when thrown on the ground, will summon a portal to the Eye of Cthulhu's domain. He, again, continues building his observatory while you work.
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Next: Kill the Eye of Cthulhu
Upon throwing the Concoction to the ground, swirling stars crash down where the bottle was broken, and forms a portal that can be entered to travel to the Eye of Cthulhu domain.
If you fail to kill the Eye and return to the overworld, Garrick can be given 5 Lunar Shards to create another Lunar Liquid, and then Eldric can be given an additional 5 Lunar Shards to create another Lunar Concoction, until the boss is defeated.
Next: Talk to Eldric
Eldric thanks you, stating that the skies are safe. The observatory should be fully built by the time you come back, and to come back if desired. The quest ends at this point.