Ancient legends speak of a being that dwells beneath the bedrock. A being so ancient that it witnessed the birth of the Nether and the End, so old that even for the first inhabitants of the Overworld, it was an old tale. A being said to have shaped the first golems and the earliest layers of the world... A legend so ancient that even its records have been lost to time...

But those are just stories, tales from ancient times, it's impossible for something like that to exist...

Unless it does... and it has always been there...

Primordial Earth Dragon summoning a Nether portal, an ancient creature that opens the doors to a new world

It's just a Nether portal, but it's made to impose and leave a mark wherever you decide to place it ;}