Ossia is a continent in the Prime Innerlands of the Realm of Shyish.
It was covered in gravestones and graveyards as the ghost-souls of those who had died in the land of Fleizch came here with an echo of their grave. They in turn were often visited by their living relatives. 
In the Age of Myth a place of peace where the dead and the living worked together.
The Age of Chaos sees the forces of Nurgle invade, the Plaguefather seeing the plains of endless graces as suitable extension for his renowned garden. Ossia has few warriors to contend with them and the living population that survive the onslaught surrender to the forces of Chaos. The land becomes stale as citadels of the new overlords spread across lands. 
By the time the Stormcast Eternals arrive in the Age of Sigmar, the underworld is dark and haunted shadow of its former self, even when stranglehold of Chaos is broken, the few leaders of the living state the aid has come too late. 
Following the Necroquake, Ossia begins to drift down the Shyish Nadir and the underworld becomes even more dark and terrible, armies of Nighthaunts arise and fall upon the remaining forces of Chaos - but only a few of the living join them - many are half dead already.
- 1: White Dwarf June 2018, Poster map of the Mortal Realms
- 2: Age of Sigmar: Core Book (2nd Edition) The Prime Innerlands, p98-101