In Elden Ring there’s a hidden underground area called Nokron Eternal City. The entrance to the area is completely unmarked on the map so unless you know it exists it’d be hard to find. There are several hidden bosses in there. Nokron also leads to another area called Deeproot Depths. This guide explains step by step how to get to Nokron.
Step 1: Defeat Shardbearer Radahn
First, you will have to defeat the main boss Shardbearer Radahn. You need to go through the portal at the start of the bridge leading to Redmane Castle, in the south of Caelid, this teleports you into the castle. Talk to the announcer and, take the elevator behind the church to find another teleporter bringing you to the boss fight. Only AFTER defeating Radahn the Nokron area will become accessible.
Step 2: Enter the Hole at Fort Haight
After Radahn is defeated, a hole will open up in the ground along the road north-west of Fort Haight in Limgrave. You can see it easily be the rocks floating in the sky above it. This hole only opens up after Radahn is defeated! Simply drop down the edges of the hole in a place where the fall won’t kill you. Then ride down the cliffs and jump down, you’ll enter an underground area which is Nokron.
That’s all there is to entering Nokron Eternal City. There is another hidden area inside called Deeproot Depths, after defeating the Valiant Gargoyle. There are also several braziers you can light up to fight a hidden boss.
For all other secret areas check out the full Elden Ring Wiki & Strategy Guide.