Before I go into details, If you want to check the fight out and where to initiate the fight:
This boss originally was inaccessible in the game until the community managed to solve 3 riddles and post their answers on a certain Twitter account.

On top of it, it required to gather a special craftable egg and 5 hidden fragments in the game by solving another galaxy brain riddle, and then looking for completely invisible enemies in the overworld that drop them.

Here's a map with the fragments and egg ingredients (otherworldly flesh) locations: https://imgur.com/a/GH2OHc2
Use the tome you get from the console to kill the invisible enemies and collect their fragment.

Before you go on the egg hunt, you must obtain ending A first, and also have less than 100 WILL stat. This stat increases whenever you die, and can be reduced per killing a heroic boss for the first time.
Follow this guide on how to craft the egg :
go to the location at the beginning of the first video above to start the fight.

Big credits to Damajen for taking his time to make this very detailed map: https://drive.google.com/drive/folders/1Lb9SW_DxTv3D4QOfJLqi8zieLP2Uo3Ad

Check out his channel too: https://www.youtube.com/c/demajen

That's about it.

Have fun!