Antigonus is one of the many World Quests that players can encounter in Enkanomiya. It’s part of the overarching World Quest called Collection of Dragons and Snakes, but can be started before the latter. In particular, Antigonus touches on the existence of Jibashiri, which are influential figures that have to do with upholding the law.
Start this World Quest by heading to the location marked in the image below. Make sure that your daylight setting is set to Evernight. The area is pretty obscure—head to the nearby Teleport Waypoint, then jump down until you find a small clearing with two afterimages labeled as Suspects.
Once you’re there, speak to the afterimage named ???, who is later revealed to be named Oogura. After, speak with the afterimage beside him—later revealed to be named Ooshima. He’ll tell you to find the third guy who was locked up with the two of them: a Jibashiri apprentice named Antigonus, or Antei for short.
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The quest will direct you to the Serpent’s Heart. Once you’re there, head to the area marked up in the image above to find a Transgressor named ???, who you later discover is Antei. He’ll tell you about having been wrongly accused of stealing a library book.
Follow the quest marker to find the nearby Ruin Grader. Interact with it and give it the Mechanical Core from Antei, who had dealt with the creature in the past. Defeat the Ruin Grader and head back to Antei to complete this World Quest. You’ll obtain the Quest Item called Before Sun and Moon, a forbidden book that’s part of the Collection of Dragons and Snakes World Quest.
Check out our guide on How to complete The Trail of Drake and Serpent in Enkanomiya Genshin Impact to unlock more of the region.