Unagi
Unagi is the eel in Jolly Roger Bay. He is also the final boss of the New Game +, appearing for revenge in Unagi's Lair after Bowser is defeated.
Plunder in the Sunken Ship
When you select star 1, Unagi will be in the sunken ship at the bottom of the level, blocking the entrance to the interior of the ship. To lure him out, Mario must swim back up to the surface, then he may swim back down to enter the ship. If Mario does not go into the ship, Unagi will go back into the ship and Mario must lure him back out again in order to get access to the area again.
Can the Eel Come out to Play?
When you select stars 2-6, Unagi will be hiding in a hole on the side of the main pit of the course. For him to leave, Mario simply needs to get near Unagi. It will have the star on its tail and Mario will need to touch that star to spawn it and make it collectable. Likewise to the first star, Unagi will eventually swim back into the hole after a while.
Hell or High Water
After defeating Bowser in Bowser in the Sky for the second time, collecting the Grand Star and choosing to take the Red Path, one last painting takes Mario to Unagi's Lair.
Unagi will have the Billion Star on its tail, increasing its power to unreasonable levels. Mario needs to swim alongside Unagi and attach mines along his body to weaken him. Unagi can only be defeated after he is weakened, with three successful eye stabs. Defeating Unagi will earn you the Billion Star and True Ending D.
This final stage is notorious amongst players for being the most difficult challenge on the entire Nintendo 64 console, including Superman. On May 6, 2022, an incident involving this level put speedrunner bramp into a coma for five months.
To date, this stage requires 108 A button presses to complete. TASers have theorized that up to 13 A presses can be saved by using the Schrodinger's Mine trick.
Kungulus Meters Down
A chasm opens up in the ground filled with water.
After swimming straight downwards for a considerable, but unknown, amount of in-game distance units, Mario will reach a Parallel Universe that is glaringly different than the others, even if that is NOT how PUs work at all, which only adds to the SPOoOoKY factor.
Here Unagi's texture is glitched because at high float-error, coordinates of 3D textures glitch out complete-fucking-ly, causing every triangle in a 3D model to spaz out all over the place. The result is Unagi looks like that thing straight out of your nightmares: MLB The Show 17.
However, speedrunners have gotten over this fear by just playing the game so fuckin much that they got bored of being scared of anything anymore, also called Exposure Therapy, where target patients (speedrunners, "fastest marios") have their naked bodies exposed for billions of twitch viewers to gawk at.
To defeat Unagi, you just need to grab 53 underwater coin rings with no time limit. This is hilariously easy for a casual player and RTA runners, but TASers have so much trouble attempting this optimization problem, that stocks in TASers dropped to the floor, causing another major recession. Every TASer's hands now has carpal tunnel. On one notable incident, Marge Simpson ran her computer through an infinite time loop, only allowing escape once the swimming has been optimized. 1 planck time unit after initiation, the space where Marge's computer once was burst into a supernova, and simultaneously spread an unknown species of carnivorous moss in a 5 mile radius. Marge was not found at the incident location, and the RTA Democratic Party declared her a terrorist.
Infinite Abyss
Next time the player turns the game on, and Mario spawns in the castle grounds, everything is submerged. It is about now that people start to notice Mario can breath underwater, so none of the water is really a hazard except for the fact that Mario cannot BLJ or squish cancel or even ground pound! When Mario swims high enough, he will see that there are infinite castles that loop in an endless grid in 3D space, which is a brand new, novel concept that I just made up. This is the Infinite Abyss, and every direction leads nowhere. There is no escaping the abyss. Everything is submerged. Your world is no more "real" than all these other ones. Unagi has not been found here so far, but its sounds are heard in the distance. There is nothing found in this area, no stars, objects, or enemies. There are portals in the walls that link to each other in a seemingly random pattern.
One of the portals near the start leads Mario back to the normal game, putting him back to the unsubmerged castle grounds.
This warp is performed by setting the floor triangles just below the portal to act as a trigger, so if Mario swims over it within a certain distance, the game will "fade out" the Infinite Abyss scene, and transition to the "Castle Grounds" scene. Decompilation of the Infinite Abyss is incomplete, but it is known that each castle is spaced out 65536 units in each of the 6 directions. The water level is actually finite, but a ludicrously large value, meaning players can spend a few years lowering the water level until the entire Infinite Abyss is drained. An inactive Unagi model is in the 0,-10000,0 castle, 655,360,000 units below the starting area, but it does not move or react to Mario's presence. After the water is drained, players can simply grab Unagi, and carry it around wherever they please, even to other maps. Funny enough, Unagi hurts enemies when thrown, which saves time in speedruns.