So I was trying to look this up...what’s the connection in lore between solteris and the entire expac?
If you read up the buried sea progression quest line, it becomes clear. Katta Castrum on ocean floor is protected by a magical force field dome that is powered by artifact caled Pellarus Satum. But artifact is losing power and also at same time, skies over Norrath get darker. Early on it is determined that it is just a conduit to real source of power: Solteris, Throne of Ro. Rest of progression is essentially getting to Solteris. Once there, you find out that Solteris is invaded by Mayong Mistmoore who wants to overpower Solusek and extinguish the sun.
