Dark Sun: The Scorched World of Athas
The large mahogany framed mirror beneath the ruins of Guistenal is the key. An artifact from the Green Age, it was constructed using abilities and knowledge far beyond even those of the sorcerer-kings. Knowledge that is now lost. With this Planar Gate, the ancients explored countless other worlds and planes of existence.
Dregoth moved the mirror from the Groaning City after he slaughtered all the non-humans living in hiding. The mirror now resides in the Dread Palace on New Giustenal.
Over many long years Dregoth mastered the Planar Gate and used it to travel the multiverse. It is through the Planar Gate that he was able to journey to the Feywild and the Shadowfell.
The Spire of Black Ice was constructed to guard the portal to the realm of the Dead Gods. Dregoth designed it to be treacherous for anyone, except himself and his undead allies. The visitors must journey to the bottom of the tower on the outside and then through the tower back to the upper levels.
There are many undead guarding the tower and the journey will be tiring for anyone. These gauntlets of undead appear to be the only defense. The gateway inside is simply activated with the key.
The Shadow Gate Tower contains the only functional portal to the Shadow Tomb and the Shadow Engine. The stability of the portal is questioned because of the destructive nature of the deep Shadowfell.
There are descriptions of the arcane wards placed by Dregoth around the gate, which would make any attempt to bypass the protections much easier. The protections were as much designed to contain the destructive energies as to prevent unauthorized access.
There are relatively fresh notes about the loss of the Shadow Engine. Its destruction was a significant setback to Dregoth, and his plans accelerated as a result. His notes suggest that he was able to gather enough energy, but only barely. Any more disruptions will be disastrous.