So, I might have had an idea for the tower... but, well, I ended up writing more than was strictly necessary, and not doing it in an entirely explanatory way. Could be a reason to have abandoned dungeons and magical ruins to fit the high fantasy theme, I guess? XD
Edes Athran. The Inverted Tower. A presence so malignant, so twisted, that even when confined to its own realm a shadow still looms over the rest of the world. Despite mages, priests, and simple scholars alike all agreeing on its presence and its effects, none can concur on its origin.
To some, the edifice is simply intrinsic to the world: Edel Athran was created alongside it to ensure that the intelligent races would never grow complacent, always having something to fear. Whether a nail shattering any hope that all could unite as one or a well-placed driver that the world never stagnates.
Others hold that its evil holds the key to its origins: Edel Athran stands as a testament to Marduk's sins, all evil feeding its growth and good shrinking it. A particularly pessimistic view, presupposing that the world's moral balance eternally sliding to the negative. A small cross-cultural group contends that one day, if differences can never be set aside, the weight of all the evil will push through to the normal world before shattering it entirely; Marduk annihilated by its own sins.
Many of a more religious bent propose that instead of a construction or part of the world, the Inverted Black Tower is instead a corpse, or the remnants of a power beyond that the gods can contend with. The story goes that in a time before the world was populated, one deity had power beyond the others and was ultimately killed--the reasons varying from telling to telling, religion to religion. Their power too great to quash, or their body too immense to destroy, it was sealed away from the world or locked itself away. The dead god's rage and malice poisoned their final remains, which continue to lash out against the other gods' creation.
Whatever its origins, Edel Athran's effects cannot be missed: appearing in dreams and nightmares often, and its malice warping magical creations. Spells go awry as whatever affixes it to this realm shifts, a moving region where the evil is strongest. Spells cast are corrupted, enchantments pervert, magical intelligences are infected by its malice. An unpredictable region whose focus can bring down empires in a night, and whose vast fringes slowly corrode untended magic and thwart apprentice spellcasters' attempts, keeping magic from ever being something truly effortless.