Pitch Black Glacier

"Many eons ago, all of the world was frozen over with a great, catastrophic force, and the sun never once managed to melt through the ice. One day, the great Gold Dragon, Honglong, thawed out the world from the ice, but was unable to unfreeze the northern lands, in which something even he himself is no match for dwells; the Ancient Guardian Beast known as Xanuu, and unholy fusion of a World Tortoise and a Yamato No Orochi. With it's extreme powers over Ice, it is capable of easily causing it's enemies to perish in absolute zero temperatures, and then revive them as frozen soldiers to command against larger threats to itself. Xanuu alone is a terrifying opponent to face, as it's summoning magic allows it's immature form to grow into it's mature form and tear apart enemies with the eight heads of it's serpent half, no matter how big they are compared to itself. No evildoer had ever witnessed Xanuu and lived to tell about it, so Xanuu's existence is nothing but mere myth and legend... until Harena herself is forced into a confrontation with the ancient deity of the cold tundra."

Design Notes:
-The first foreshadowing of Xanuu's existence comes into play in Chapter 3; Xanuu, in the ruins of a mini dungeon which contains a key item necessary for the plot, is noticeably a frozen over area in a region otherwise filled with tropical jungles and a temperature not too different from South Islands or East Island, and not once does the ice ever melt. On the wall the characters read, they get a cave painting of a black mass of sorts that doesn't exactly make much sense to them, as it doesn't resemble Xanuu at all.
-Xanuu is not directly playable in Chapter 4's beginning, since you're actually playing as Tehka until Xanuu takes over as the lead character. Throughout his journey through the northern region, he finds similar paintings of Xanuu which become increasingly more clear as he progresses through the region. Each painting he comes across reveals something about Xanuu's history. In the ancient past, Xanuu was originally evil in nature, and existed solely to freeze the world over to cause the previous ice age, but was stopped at the last possible moment by Honglong's golden flare technique, which thawed out the planet and sealed Xanuu away to the northernmost region. As a result, Xanuu was not present in the battle 500 years ago when the prior Guardian Beasts fought and perished against Harena, since Xanuu was in deep slumber.
-Xanuu, due to it's backstory, is both the 1st Gen and 2nd Gen incarnation of the Black Tortoise/Snake in terms of the storyline goes, since the other three modern incarnations are all 2nd Gen, whereas Xanuu had no true 2nd Gen counterpart due to still being alive to this day.
-At the end of the introduction to Chapter 4, Xanuu is the first boss faced by both Harena and Tehka, in which you switch to playing as Xanuu and are completely outmatched by Xanuu's fighting capabilities, and the remainder of the Chapter is basically Xanuu traveling through each of the three prior regions to revive the other Guardian Beasts and completely demolishing every enemy that stands in your way in the process. Basically, pretty much the entirety of Chapter 4 is to demonstrate how freaking overpowered Xanuu is as a Guardian Beast, a power not going to be seen rivaled until Honglong shows up during Harena's ultimate demise. Note, though, that all of the enemies Xanuu will be facing would easily demolish Sirryu, Zucue, and Baywho combined with no difficulties; Xanuu's stats and elemental resists far surpass the other three to the point practically no enemy can score a proper damage dealing hit against Xanuu. Not only that, but Xanuu's equipment vastly increases Xanuu's stats more so than what the prior characters have as equipment.