After you gather the Sanctions of the three goddesses, Null's evil plan is revealed.

Speak with General Wright

Following the cutscene after you receive the three Sanctions, leave your room and there will be a cutscene.

Go to the Eternal Forest

The Eternal Forest is northwest of Hyrule Castle. If you have cleared the Stilled Northern Sanctuary, you can go west from the Sanctuary to find the forest.

There is also a waypoint to the west of the Sanctuary. After you check it to add it to your map, go north, then slightly west, to find another waypoint. From there, go northwest and there will be a cutscene.

Read the stone monument. Then learn the Ancient Orb echo. Bind to this orb and place it in the pedestal to the south of the stone monument. There will be a cutscene.

You will find the remaining pedestals in the locations that are shown on the stone monument, in a triangle shape.

Go southeast to the waypoint, then go south from there and defeat the Caromadillo. Then go to the east side of the area where the Caromadillo was, and create an Ancient Orb echo, and bind to it and place it in the pedestal.

Go southeast from that pedestal and look for a buried treasure chest. There is a pedestal slightly northeast of it.

There is another pedestal directly west of the previous one, and another pedestal directly west of that.

Go northeast from there to find the final pedestal. There is a Deku Baba on top of the pedestal, so get rid of the Deku Baba, then put an Ancient Orb into the pedestal.

Then go back and read the stone pedestal again, and there will be a cutscene.

Then go north until there is a cutscene.

Ancient Ruins

Go north until there is a cutscene. Then go north again, and there will be another cutscene. Afterward, check on the glowing spot in the wall. After the cutscene, enter the glowing spot. You will enter the Stilled Ancient Ruins.