After defeating the third shard bearer, General Radahn, a falling star lands somewhere in the Lands Between, opening up a secret region within Elden Ring. Story-wise, Radahn used his strength to hold back the constellations, so after he is defeated, they all return, causing a star to fall. The cutscene that shows this does not clarify where it fell, and the NPCs around post Radahn fight tell you that it fell somewhere in Limgrave. Here is where the falling star lands in Elden Ring.
The falling star lands in east Limgrave
The hole created by the falling star will be located in the eastern part of Limgrave, on the southern part of the forest area. The easiest way to navigate here is to use the Site of Grace to the east of it or the one at the church to the north. You can attempt to drop down from the western side, but the cliff is pretty steep and could easily result in death if you’re not careful. A good visual marker to navigate here is that there are a bunch of floating rocks in the sky above the hole that are visible from pretty much anywhere in Limgrave.
You can navigate down into the hole by jumping from floating rock to floating rock. Once inside the hole, you will find yourself in Nokron, the Eternal City. There are a couple of optional bosses down there, along with an item needed to advance through Ranni the Witch’s side quest. This is also where you can find the Mimic Tear spirit ashes, a powerful spirit summon.