The Entrance to Tokoyo is a World Quest that will aid you in your deep dive into Enkanomiya. Players will automatically unlock it after completing the prerequisite quest called Still Water's Flow.
Upon landing in the world under Watatsumi Island, you'll find a dark, cave-like place with no light apart from fluorescent plants and stones, with most of the areas blocked off from exploration. The Entrance to Tokoyo will help you cross the giant Sunfire Gate and enter a place filled with light.
Start by following the quest marker to the giant gate containing the five Key Sigils. You'll be prompted to investigate five stones that contain the same Sigil-like motifs around the area.
Related: How to switch between Evernight and Whitenight in Enkanomiya
Simply interact with the motifs to activate them. Once all five are done, head back to the Teleport Waypoint and interact with the mechanism to activate the giant gate and prompt a cutscene to play. After, cross the gate to unlock the Wonders of the World Achievement Flowing Sunfire, Also Known as Marishi.
The quest marker will continue to mark up a path for you to follow, which will end when you're greeted by a door. Interact with the nearby mechanism to activate it and to prompt a cutscene to play, which will open the door and take you to the hidden civilization deep within Enkanomiya.
If you haven't already done the prerequisite to enter Enkanomiya, check out our guide on How to complete the Enkanomiya prerequisite quest: Still Water’s Flow in Genshin Impact.