The Tower of Oblivion, also known as the Sinking Temple, is a ruin of a tower in the kingdom of Othiria that was built and inhabited by the archmage Selderin Vermth in his last years.
Once located on lush planes of Skarbury, the landscape was transformed into a swamp from one day to another, and Selderin hasn't been seen since. Attempts to enter the tower have all failed, with most not coming back, and those who do having their minds wiped.
Inside the tower
The tower is separated into dozens of pocket dimensions of existences, with travel possible between them using portals. Above each portal is a small glass ball with elements of the target plane. These balls can be removed and switched out with delicate manipulation.
Protectors of the dimensions
Each plane will have different challenges to combat potential intruders, with a strong focus on defeating magic users, who Selderin viewed as most likely to try and steal from him. Adventures are captured with anti-magic shackles and groupes are split off when traversing portals. Adventurers who are captures will either die, or if strong enough will survive physically, but their minds will be whipped and they will become part of the tower's inhabitants who maintain the various planes in order, and fight off intruders.
Selderin was also interested in mechanical creatures, and created huge automatons that will track down adventurers and imprison them. These are created and repaired in the Workshops, using souls harnessed in the Soul Mines.
What keeps it all together
Selderin started building the tower and it's planes with the help of the elven druid Giric Tousseau. Seldering trapped her into the Soul Mines to power the pocket dimensions for eternity.