Dwarven Gate

The Dwarven Gate was a planar portal beneath Castle Sengir that led from Ulgrotha to an unknown plane. Baron Sengir wished to conquer Ulgrotha, then march his troops through the portal to take whatever was on the other side.

On the eve of the Great Mending in 4500 AR, the planeswalker Leshrac saw a great army of vampires, marching at the order of Ihsan's Shade. The Baron was last seen at the head of this army, walking through the portal. Wherever is on the other side is surely doomed.

Since the Mending the portal is no longer functioning. Mark Rosewater hinted it might have led to the Tolarian Academy on Dominaria.