This is the NetherEnd Dimension, where you can explore a nether like end and still find what is in the nether and in the end, both dimensions have been combined into one making it faster and more convenient to gather loot

To get into the NetherEnd you must find a NetherEnd Stronghold and defeat the Gaoler (Mob Boss) and obtain the final Portal Frame and place it with an Eye of Nether

Once you enter the NetherEnd you are trapped and can find Zulkis' Arena and defeat him to obtain the Nethendite Armor and Tools and once you defeat him you may finally leave the NetherEnd through the portal

Gaoler (Portal Guard)