@MaeAs you've said, Daethaneus destroyed a continent, which is now cursed with his energy. As eras pass, the energy fades as magic does, so when they reach the Medieval Era, it's still there, but weaker; Renaissance-esque era it isn't very noticable; and Modern Era it's almost undetectable. His Immortal soul is drawn to the centre of the land (the largest city in later eras), and will always return there if his mortal coil is destroyed. His name is probably well known but his worship is extremely rare, those that do are seen as cursed, psychopathic warmongers and murderers.
His name is used as a curse synonymous with any form of Death or War. A phrase like 'May Daethaneus follow your steps!' would be like wishing someone to be followed by death and hatred, or 'Daethaneus smiles upon you.' would be calling someone a murderer and warmonger. In the modern era, it's basically saying someone is in a cult.
Also, his Thyt'r still roam the lands, and (with your permission. for plot reasons) some of them have travelled to other continents, searching for their lord and master.