Players can find the Secret Fusion Shrine in Steal An Egg inside the Angels/Demons Map. Clip through the waterfall opposite the large gate at the end of the landmark to reach the fusion location.
In Steal An Egg, players can now fuse their Light & Dark Divine/Eternal pets to make new Eggs. While everyone understands the ritual, many are confused about where the fusion shrine is located. So, if you are one of them, don't worry! Here is a detailed guide to help you find it quickly.
Where is the Light & Dark Fusion Shrine in Steal An Egg?

The main reason everyone struggles to find the Shrine location in Steal An Egg is that it's a secret area, and you can only enter if you know the exact location. First, increase your speed and walk straight to the Angels or Demons, which is currently the last landmark in the game. This will take anywhere from 10 seconds to 1 minute, depending on your current walk speed.

Once you reach the location, you'll find a gate on the right, as shown in the image above. This will be the Angel's Gate if you are visiting during the day, and the Demon's Gate if you are walking at night. Right opposite this Gate, you can find a waterfall. Enter through this exact waterfall to reach the fusion area. Like the Gate, the Waterfall will look blue (normal water color) or red, depending on the time.
How to fuse Light & Dark Pets in Steal An Egg
Inside the fusion area, you will have two platforms, one for the Light Divine/Eternal pet and the other for the Dark Divine/Eternal pet. Since this requires two players, you can invite any of your friends who can provide the alternate version of the pet you are holding. If none of your friends possess this, you can join the Steal An Egg Discord, where you can find other random players ready to fuse their pets with yours.

Once you have a partner to fuse with, go to the platform and hold your respective pets. Immediately, within a few seconds, both your pets will start rotating and fuse to form a Fused Light + Dark Fusion. Each player will receive an individual egg, which you can then hatch into some of the rarest pets in the game.
For more on Steal An Egg, check out Complete Steal an Egg Sakura Incubator Guide or Complete Steal An Egg Rift Event Guide