Gate of the Abyss

These ancient portals are a product of dark necromancy and sheer will. Lacking the precision of a Tower portal, an Abyssal Gate has a chance of accidentally transporting the traveler to a random location in the abyss.
Normally however, these gateways serve as portals to the great abyssal cities of Enoch or Atlantis
Due to the effectiveness and success of this awe inspiring design, House Aries not only uses it as a Citygate, but they have also built a much larger version to serve as an Abyssal Planar Portal as well

Credit: Arcadeous Phoenix