Gods are "divine" beings worshiped by people. Most of Vosca follows a unified cosmology with the two celestial bodies opposed to the seven Chaos Gods.
The Celestial Gods are those which correspond to heavenly bodies. There are two of them, Genosus, the Sun, who spends one day each month in his temple on Mount Genosus in Solar Illegon. There, he speaks with the Pontifatrix and may interact with his Paladins, those faithful dead of his worshippers who may return to the mortal realm. The other is Bocanadessia, the Moon, and ruler of Eternal Karthack. Her greatest servants are reborn from her own womb in new bodies as the Silver Ones.
The Seven Gods of Chaos, or just the Seven, are primordial entities which existed as long as the world or longer. They are considered opposed to the Sun and Moon but share a religious cosmology. They are primarily worshiped by the Zell.
Pagans in general do have their own gods, and these are (to varying degrees) as "real" as Genosus or Bocanadessia are. Their emblems have power, they can be communicated with in certain ways, and they have their own ways of interacting with their people. The northmen have several pantheons, the people of Lost Izagone have their own, as do the Marju of the endless steppe, and even a few holdout groups in civilized lands. The Ohanedin have their spirits, too. Worship of spirits is very common.
The Sarturi are a splinter-faction of the vikings who, in the hopes of escaping from the horrors of the Zellish invasion, attempted to switch sides to the worship of Sartur, a powerful spirit of fire, war, and blood, who was the traditional enemy of the norse pantheon.
This worked temporarily. The Sarturi were granted strength, courage, powerful magic, and the Zells weren't sure if they were chaos-worshippers or not, and as such left them alone to wreak havoc wherever they felt like it. Someone else's problem.
But Sartur didn't take on these worshippers to give them peace, he took them on as instruments of his wrath. Now the Sarturi number in the tens of thousands, and have begun to sweep back into the lands from which they fled, retaking them one bloody footstep at a time, both from their former compatriots, the Vikings who held to the old ways, and from the civilized converts to Genosus who had been advancing slowly into the vacant lands.
The Zells have started to recognize this threat as well, they simply can't ignore them. Whatever sort of old-god Sartur is, he is definitely no friend to The Seven, Genosus, or Wotan.