This is the guardian that lives under the lava in the nether. it has never been seen but some guardians that live in the sea in the overworld that look exactly like it. The real one has been hidden for years and protects its temple that it gets its powers. He tries to leave the lava but the player keeps popping up. while the player is gone, nether mobs try to get powers from the fire temple but can't get past the nether guardian. this is also part of the reason striders can walk on lava. They always kept trying to get to the temple so the guardian cursed them so they couldn't swim inside the lava.