Folios and Domains

Gods have domains and folios, but what are they?
Domains are something a god can choose to have, so long as they're tied to the god's ideals. The more domains a god has, the more diluted their power. Domains are a 'real' thing that priests can discuss as a literal concept (as are folios).
The more domains a god has, the more appealing they are to followers, but the weaker they become.
Folios are more complicated. Orcus holds the Death folio, which could be considered a deed to the "land" of death. The more folios a god holds, the stronger they become. Because Orcus holds the Death folio, a fraction of any worship of Death gets tithed to Orcus, even if the god being worshipped really hates Orcus. Orcus can be thought of as holding the copyright on death, and must be paid royalties. 
This continues on up: While Orcus holds the death folio within the Valley pantheon, it's likely that Zon-Kuthon (The LE god of pain, torture and the resulting death) holds the greater Death folio, so some of Orcus's worship is tithed up to him.
So what's the result of all of this?
One example is Kel. When Mitra was at her peak (indeed, she was so powerful that she eclipsed all other Valley gods and the Valley was on the brink of monotheism), she chose to take the mortal Kel and to ascend her to godhood. Mitra gave Kel the protection domain, while Mitra hung on to the protection folio. This worked really well for everyone: Mitra reduced her domains and thus gained power, while some fraction of Kel's worship would be tithed to Mitra. Meanwhile, Kel got godhood (albeit the diet-soda version). 
When Muir died, her folios were passed on to Mitra. This is pretty standard, as folios are passed on or inherited when a god dies. If no one is around to inherit them, they instead pass up to whomever holds the greater folio. This is why some of the greater gods (like Pharasma) are worshipped in the Valley; whatever Valley god once held those folios is long dead, and they (along with their followers) transferred over to the greater god.
Where this gets awkward is with Orcus and Tsathogga. Orcus holds the Death folio, and Tsathogga holds the Destruction folio. Orcus has the Destruction domain, while Tsathogga has the Death domain. This means that some of Orcus' worship is passed to Tsathogga, and vice versa. Thus the gods fates are linked, as one gets stronger, so does the other as they are literally sharing worship. 
"Surely then, even though they have a symbiotic relationship, if Orcus or Tsathogga died, the other would likely inherit the folio, and greatly strengthen the survivor's position". Indeed so, and that is exactly why you get holy wars, and the love/hate relationship between the two demon lords.