Basic Lore: The Tower of Malice was created a very long time ago by the Holy Empire, a powerful theocratic nation that formerly dominated most of the continent. The Tower was used as a place to put really bad things so that they couldn't spread their corruptible influence or be used for evil purpose.
Finding the Tower
Finding the Tower of Malice is no easy task, as it was built in an auspicious but also relatively remote location, and most of the available lore on how to find said location is in poor condition. The Tower is nowhere near any major population centers, and is many days travel from the nearest settlement that could be called a village. It is located in somewhat predictably forlorn weathered mountains, the sort that are shrouded in clouds, and have valleys that hold banks of fog even during the hottest parts of the day.
Band of Nomads - the hinterlands are not uninhabited. A band of nomads crosses paths with the PCs. The nomads are ranchers, following their livestock as they graze across the scrubland. They can offer aid, dire warnings about the evil lands, break bread, and offer some basic trading. They do not know of the Tower, but know of the place where you don't go, because of the evil spirits and the cursed soil
Band of Bandits - that's a nice mount you've got there, hand it over and we won't cut your legs and you can walk out of here.
Dire Beast - corrupted by the miasma of the Tower, mundane animals can be warped and grow large, aggressive, and clever. They are drawn to sentient beings and hunger for their flesh.
Wandering Undead - rather than a roving band of zombies, they are raggedy shamblers, nomads and bandits, fallen clergymen and explorers who died too close to the tower, and now they are cursed to wander.
Intermediate Lore: The Holy Empire built the Tower of Malice originally as a prison to hold dangerous criminals, specifically those who were immortal and unable to be killed, such as liches, demigods, and devilspawn. Such beings were reduced to their weakest states before being interred into the tower, such as liches being reduced to just their phylactaries and then said object being encased in a vessel of silver, and then being placed in a warded vault in the tower. Devil spawn would be crucified, their organs vivisected and pinned to the walls, and left in a state of permanent torment, with their regeneration keeping them perpetually alive.
Intermediate Lore II: The Tower was also used as a Vault of Inequity, a place to keep evil things. Rather than creating an arsenal of evil that a necromancer, hellmage, or some other fiend could conquer, the items were taken to the tower basements, and destroyed. Their remains, like the skeletons of the less immortal prisoners, are still there.
The Tower Walls
The Tower of Malice, sitting on it's stone outcropping, is surrounded by a prison wall, of substantial height and thickness. It is the sort of fortification that could withstand a great and terrible siege, with one exception. The ladders and crenelations face to tower, and the wall itself faces not outward to defend, but inward, to contain. There are no gates, as the wall was built around the tower after it's completion. Crossing the wall requires a very difficult search to find the Path of Forbidden Mysteries, or the PCs will have to rappel down the face of the wall to get in and out.
Advanced Lore: The Path of Forbidden Mysteries is the walkway under the wall and into the tower grounds, so that clerics and crusaders could carry cursed items and prisoners into the tower without introducing weaknesses in the wall. There are powerful magics around the wall itself, preventing anything from flying over, teleporting in or out, scrying through or out, or otherwise using magic to enter or escape the tower. The Path entrance is a shrine several hundred feet away from the foot of the wall. It slopes down to become and underground passage. There are dozens of wardings and protections that the PCs or clerics would have to pass through (one way, anything evil may enter, but it cannot leave). The exit of the Path is another Shrine, dedicated to the supreme pantheon of the Gods of Light and Law, and beyond that, the Tower courtyard.
The Courtyard of the Tower is not empty, there are dozens of structures, including a large abbey with a consecrated cemetary, barracks, and the other buildings consistent with an imperial outpost. They are made of stone, with slate roofs and are very well made. Despite the passage of a great deal of time, the buildings are all in surprisingly good shape. There is very little overgrowth, as the majority of the courtyard is paved with flagstone and cobblestone.
The Trapped - over time, nomads and other people have entered the tower, seeking its treasures or just shelter. The wards prevent evil and chaotic beings from leaving, and they were trapped inside. They are few in number, paranoid, gaunt, and most have long since gone mad. They subsist on rats and the remnants of imperial gardens on the grounds.
Undead - only active at night, horrific and bizarre undead climb out of their crypts and pits and roam the courtyard. They hunger and they are vicious. They do not rise as soon as the sun goes down, but rather are in a dormant state until something living gets close enough to rouse them. This could happen during the day, with the undead primed to emerge, or during an evening trip to the latrine to discover that there was a rib-maw zombie slumbering in the pit.
Sentient Scavenger - warped by the influence of the tower, animals grow large, and demi-sentient. Kobold sized rats, ravens with grotesque danging organs, vultures that cackle and sing funeral dirges from the top of the wall.
Advanced Lore II: The Keepers of the Tower themselves were prisoners, typically members of the clergy who had committed transgressions against God and Emperor, and their atonement was to serve at the Tower Garrison. This was a life sentence, as most were marked such that they couldn't pass through the Path of Forbidden Mysteries. They lived in the courtyard, tended the grounds, walked the vaults, and eventually, when their time ran out, were buried in the consecrated cemetery. The cemetery is the only safe place inside the courtyard, and remains inviolate.
There is a single entrance into the Tower and it is guarded by a pair of massive golems, one a chalcedony Goddess of Light, the other a jasper God of Law, both armed with golem sized enchanted weapons. The Goddess of Light bears a Holy Spear made of gold, wrapped in powerful enchantments. The God of Law has a Holy Sword made of silver woven with dozens and incantations. These guardians are as much jailers as they are defenders.
Advanced Lore III: the Golems of Law and Light can be commanded by an Imperial Bishop or higher ranked clergyman, or by the Holy Emperor or someone bearing his Ring, Scepter, or Diadem. The Imperial Clergy is gone, so there are no more ranking clergy who can command the golems, but the Imperial Ring, Scepter, and Diadem are still around. Suitable locations: lost in a dragon's hoard, a status symbol of a modern king or queen, a relic in the cathedral, and so forth. The golems have been used twice. The dracoliche Glau the Immortal breached the wall in an attempt to make the tower it's lair. It was beheaded, dismembered, and imprisoned in the tower. The starstruck mad archmage Elborath the Desolate allowed himself to be captured and once inside the walls attempted to call his star dwelling masters, but was quickly dispatched by a spear through the chest (which removed his head, and most of the left side of his body.)
The Tower of Malice
The inside of the Tower of Malice is a place of utter terror and stark nightmare. In it's long period of neglect, it has become encrusted with evil and overgrown with corruption. The spiritual corruption of the things it contains have spilled over into even the very air. The stone seems to rot and crumble, organic matter is decayed and festering, but never completely consumed, and nothing dies. Death would be a release in such a horrible place.
Powerful Undead - undead raised by the evil of the tower and fed bloated and strong on it, such creatures are nigh unstoppable engines of death and mayhem. (Nemesis ala Resident Evil)
Ghost - unable to rest or depart, ghosts wander some levels of the tower as thick as fog. Such levels are incredibly dangerous, as the ghosts can draw out life force or drive the living mad. Powerful ghosts can inhabit and animate things like corpses and suits of armor, while very powerful ghosts can take over the bodies of the weak willed and weak spirited living.
Horrible horrible things like a pit of undead infants, vomiting statues, and cursed cisterns
A sentient spell - most often the free roaming magics of imprisoned liches, immortal archmages, and mad demons, these entities appear as glowing spheres and cracking bolts of magical power. Wither limb, enervation, Power Word: Kill, Raise Undead, Curse, and the other vile ephemera that an imprisoned magical being would belch up at random.
There are hundreds of cells inside the Tower, while many are general purpose, fitted with iron bars and basic wardings, the worst are buried deep in the earth, custom built for their occupants. The basic cells are littered with skeletal remains, some of which still move and chatter when the living come close. Or their occupants, centuries dead and mummified, reach through the bars and grasp at the living as they walk by. Opening these cells is pointless, their contents have no value (no gold, no magic items, and as basic undead, beginner level xp)/
The Eternal Vaults
The Eternal Vaults were built to last until the end of time, as they contain the worst horrors of the Holy Empire's time. There are at least a dozen civilization ending supervillains contained in the Eternal Vaults, liches, archdemons, a dracoliche, and so forth. Opening their vaults would be foolhardy, as most, even in their suspended, crushed, powdered, crystallized, permanently frozen or otherwise deposed states, would be able to pulp all but the most potent PC. Even venturing into the Eternal Vaults is foolish as such monstrous things can corrupt the PC, changing their alignment, and preventing them from leaving the Tower because of the wards protecting it. If a group of PCs did enter the Eternal Vaults and escape, they will be forever changed by it, dropped a few sanity points, picking up a few nasty flaws like Nightmares, Doomed Fate, etc.
Advanced Lore IV: The Eternal Vaults were eventually realized to be a mistake, as such evil concentrated in one place created a spiritual stain, and started corrupting the environment around it. After this was realized, no new inmates were interred in the Eternal Vaults, the containment wall was built, and the Great Furnace was built. Prior to this, the relics and cursed artifacts held at the Tower were simply locked away, but these evil things started growing in power, due to simple proximity to the Eternal Vaults. The Furnace was built, the artifacts and relics were fed into it, as were the bodies of the criminals and monsters who died inside the wall. The remaining ash was dumped into a pit in the very bottom of the tower.
The Great Furnace and the Imperial Crematorium
The Great Furnace is a triple jet blast furnace that draws on three sources of tremendous heat. The Furnace has an Elemental Flame jet, a Celestial Flare jet, and an Hellfire jet. The three flames create as close to perfect combustion as can be reached and thus far nothing has survived being passed through it. The clerics spent months feeding cursed magic items, baleful weapons, unholy books, and the bodies or thousands of Imperial prisoners through the furnace. It was called the great cleansing, and near the end, many of the furnace tenders threw themselves into the furnace, overcome by their own souls and the deeds of their hands. Though long dormant, three magic users could easily restart the device.
The ash piles in the crematorium are a testament to the destruction and death that was dealt by the furnace, and accordingly, the ash pit is haunted. Few have the spiritual and emotional fortitude to walk through the pit without being overcome by depression and actively suicidal actions.
Stop the Villain - a powerful villain has decided to strike the Tower, and release it's prisoners onto an unsuspecting world. To this end, the villain has acquired the Asdas and will rekindle the Great Furnace and cast the Fellstar into it. The energy and evil released by the destruction of the Fellstar should be enough to break the wards around the tower and to crack the tower itself. The PCs have to stop him before he releases the horrors of the Old World.
Question the Devil - an ancient prophecy is starting to come true, but parts are missing, or are written in an unknown language. There is one who can read it, but said archmage is a prisoner in the Eternal Vault. The PCs must dare the Tower and the Vaults, then Silence of the Lambs convince a thousand year prisoner mad evil archmage to give them a hand without ending up as the agents of the apocalypse themselves.
Holy Pilgrimage - a group of Clerical PCs have been given a holy quest to go to the Tower and resanctify the cemetery and refresh the wards around the walls. It's something that has to be done once a century or so.