Angle Eraser consists of 3 Beastly Rifts in Beryl Region that you need to concur. In each location, you need to complete a challenge of fighting Rifthounds and get a Precious Chest.
How to Get Angle Eraser Achievement in Genshin Impact
The map above shows the location of 3 Beastly Rifts in Beryl Region, Fontaine, that you need to go and complete the challenge.
➥ Teleport to Elynas south waypoint and glide south until you see a patch of purple land. Only after you defeat the first wave of Rifthounds will the challenge start (indicated by the Beastly Rift title like in the image above).
➥ Teleport to Elynas south waypoint (same as the previous waypoint) and glide northwest for the Beastly Rift.
➥ Teleport to Elynas northwest waypoint and look to your east for the Beastly Rift. If you already completed the Book of Esoteric Revelations quest, then you have most likely completed this challenge because the Precious Chest from this challenge will give Enigmatic Page (III) that was required to complete the quest.