Mortannis is a city in the Realm of Shyish.
The city was ruled by Queen Ahalaset, a Soulblight Vampire and whilst not a direct threat to nearby Nulahmia, Neferata was always wary of either gaining too much power. During the Age of Chaos she allowed its queen to form an alliance with Lord Nagen of the neighbouring city-state Nachtwache to defend themselves against the invaders.
Once the Age of Sigmar dawned and saw much of the realm reclaimed from Chaos, Neferata was no longer willing to tollerate the ongoing alliance and matters came to a height when she was invited to a ball in the city - the Dance of the Skulls. 
The city itself is surrounded by mountains.