Alright, bio time! Now, it's just me, but I like doing villains/antagonists first, so let me introduce them:
Big Bad
Sachin Prabhu
Meaning: (Sanskrit) Sachin = "Pure" Prabhu = "Mighty, Powerful, Master"
Background and abilities: A member of the Underground Civilization and the 27th Generation of the Mark of the Sun. Being such a high generation, he has godlike magical ability, which he and his predecessors have used to allow the Underground Civilization to thrive despite it's dismal circumstances. As such, the Prabhu family line has been a line of unquestioned God Kings, whose word is law and every command is obeyed. He is worshipped, served, and protected by every member of his Civilization equally in order to prevent the mark's curse from stealing him away from them. Apart from leading the Underground Civilization, his only other responsibility is using his grandiose power to raise the Great Tower from the depths of the earth and into the sky, so that he may collect more light energy to feed his Civilization's energy demands. He is incredibly proud, but also wise, not only from the teachings of his father, the 26th Gen Mark of the Sun, but also from the fact that he has supreme knowledge of all things from being attuned to the sun, and that since the sun knows all that which it shines upon, so does he. He knows the above ground races are not to be trusted or dealt with, and so has brought his greatest armies and warriors up with him to slaughter them should they attack. He has also brought his son along with him, to teach him the ritual that he too will someday have to do.
Appearance: Unlike every other member of the Underground Civilization, which are pale-skinned due to being unable to withstand direct sunlight, his magic grants him immunity to the harmful effects of sunlight, and thus upon obtaining the mark he bathed in the light from the light-magic devices in his Civilization, and has a very deep, dark tan as a result. He still retains the white hair of his race though. Has pitch black eyes from his magic completely absorbing light through them instead of letting it bounce off. When encountered at the top of the tower, he is completely nude, except for a loincloth. Is extremely muscular, because he looks more awesome that way.
Tropes applied: God Emperor, Loincloth
The Underground Generals
The generals of Sachin's Armies are those that have earned his trust and respect the most, and all have been granted special magical items, crafted by Sachin Himself, in order to annihilate any human threats.
Savio Seth
Meaning: Savio = "Bright, Clever" Seth = Egyptian god of Chaos and Sand
Background and abilities: Savio is the most respected and intelligent of the four generals. Unlike the other generals, who rely on violence, he uses wit, strategy, charisma, and manipulation to subdue his adversaries, and Sachin, seeing this, plucked out his eyes and replaced them with powerful magical items that allow him to read his opponent's minds by looking into their eyes. This is a closely guarded secret however, and only he, Sachin, and the other generals know of his advantage.
Appearance: Like every other undergrounder, pale with white hair. However, he goes out of his way to wear colorful and aesthetically pleasing outfits. Since his eyes are magical items, they will occasionally shine or glow when being overused, giving away that they are special.
Tropes used: Magnificent Bastard, Glowing eyes of doom, affably evil, Telepathy, The Dragon.
Darnell Vahan
Meaning: Darnell = "Hidden" Vahan = "Shield"
Background and abilities: While the other generals wear armor, Darnell is special in that not only does she wear full plate armor over her entire body, but the armor she wears is also embroidered with magic gems, given to her by Sachin, that allow her to emit a powerful forcefield. These forcefields, combined with her natural obstinance, has given her the nickname "Unbreakable Wall Darnell" among the other generals. She also has a magic sword that will repair itself if broken. While she will actively pursue intruders, she prefers to fight in a special part of the tower made just for her, which has walls that are covered in spikes and spears, and she can push her enemies into them using the forcefields.
Appearance: Like any other undergrounder, not that you'd know, since her armor covers her up completely. Since her image is concealed by her armor, those encountering her may not know she's a woman. Her armor is black, and is covered in shining golden gems.
Tropes used: Samus is a girl, Black Knight, Barrier Warrior, Combat Pragmatist
Chryses Nerva
Meaning: Chryses = "Golden" Nerva = "Strength"
Background and Abilities: Chryses watches over the middle section of the tower, which houses the furnaces, barracks, and reservoir. Was originally a servant that served Sachin directly, but an accident occurred, and was severely maimed while protecting Sachin. He had lost his arm and snapped his back in two, and seeing this, Sachin repayed him restoring him back to health with magical items. He gave him a stronger back, and a much more powerful arm. With his new arm, he has strength unlike any mortal, and using this strength can overpower nearly any opponent. In dire situations, he can overload his arm to deal one final, incredibly powerful blow which leaves his arm broken and shattered. Sachin, however, had the foresight to make the arm easily repairable. There are also a number of smaller magical items that keep Chyrses in top health, and emit a very strong aura that protects him from weaker magical attacks.
Appearance: Like any other Undergrounder, but his arm's veins are golden and shine brilliantly when in use.
Tropes used: Power fist, Artificial limb, The Pesci, Desperation Attack, Battle Aura
Agrippa Durai
Meaning: Agrippa = "Wild" Durai = "Leader"
Background and abilities: Watching over the security of the entire tower, Agrippa is the oldest and most experienced of the four generals. Since Sachin himself can't be bothered during his meditation, Agrippa is tasked with making sure things don't become hectic in the tower. To do this, he's given several magic items, one which allows him to communicate with anyone else in the tower at any time, and another that allows him to control the various beasts and monsters that were captured from the darkest pits of the earth, and a third that allows him to shift around the rooms of the tower to some extent. Ontop of this, he also has the most experienced and violent warriors in his personal army to do his bidding. Agrippa himself, while known for his mean streaks, is very conservative and knows when to run away from a losing fight.
Appearance: Quite taller than you average Undergrounder. Has wrinkles, and is balding.
Tropes used: Old Master, The Beast Master, Screw Politeness I'm a senior
The Underground Warriors and Beasts
These adversaries are notable for being exceptionally difficult, and thus have been either forcefully or willfully enscripted into the Underground Army to make use of them.
Jundasa
Meaning: Jun = "Talented" Dasa = "Servant"
background and Abilities: Jun, simply put, is a prodigy. Born with combat talent and intelligence that absolutely no one before or since has ever been able to rival. He's also hundreds of years old. From what can be gathered, he attempted to assassinate the 21st Generation Mark of the Sun, and as punishment he's been forcibly kept alive all these years by having his entire body reconstructed with magical items so as to live forever, and then placed under a spell so that his every action is controlled by either Agrippa or Sachin himself. While there have been others that have rebelled, Jun is receives special punishment since he's the only one to have gotten close to succeeding. This near-success is attributed to him discovering a shared weakness among all magic users that he's never shared with anyone, and using that knowledge to make him invulnerable to magic. His usefulness as a slave is evident from him having killed every single Marked One that he's ever been set against.
Appearance: Appears to be someone who has been mortally wounded several times over, with deep gashes over face, neck, and body. Wears an old tattered soldiers uniform. Doesn't bleed.
Tropes: The Gift, Mind control device, anti magic, Who wants to live forever, fate worse than death
Skadi
Meaning: Skadi = "Damage"
Background and Abilities: Skadi is a giant hound-like beast, which is controlled by Agrippa. There are many mysterious caverns that the Undergrounders find while exploring the Earth, and Skadi was found inside one of them. She is huge, and incredibly vicious.
Appearance: Large, with white coat and black mane
Tropes used: Angry guard dog
Tisis
Meaning: Tisis = "Vengeance"
Background and abilities: Also found in the ancient caverns of the Earth, the Tisis is an aquatic creature the inhabits the Tower's reservoir and is controlled by Agrippa. It's biology is alien even to those that study it closely. It appears to be a single creature, but also appears to be many combined into one. Whatever it is, it is incredibly aggressive, happily leaping onto the shore to attack intruders that wander too near, and will leap out onto ships and attempt to kill as much as possible before diving back into the water. Injuring it seems to have no effect, and even bisecting it produces no results as the halves will regroup into a singular entity again.
Appearance: Alien, like a giant black shark. Has very sharp teeth and tail.
Tropes Used: Everything's worse with sharks
Sorry if I was a bit wordy. I have some more bad guys in mind, but I'll write them up later.
EDIT: Added Beasts and Warriors.