Spoilers for Mummy: the Curse ahead, relating to the Deceived/Lost Guild.
Book of the Deceived reveals that the Shan'iatu of the Sixth Guild were denied the shot at godhood that their peers took, and only survive as splintered, insane soul-fragments (known as temakh) that cannot exist without an (im)mortal host. The Rite was the Shan'iatu’s attempt at becoming the Judges of Duat (essentially, gods of death). If that failure produces these strange parasitic spirit beings, then we have some interesting threads to follow.
Geists are the result of a mortal ghost doing /something/ to become less “the remnant of a human being” and more “an embodiment of an aspect of death.” Geists crave a mortal host to act as their hands in the world of the living, binding themselves to Sin-Eaters. Sound familiar?
Geists and Temakh seem to be very similar creatures with different (but related!) origins: creatures associating themselves with elements of death who require a sapient host.
Add onto this that the Temakh crave cults to worship them while Geists grow in power upon adding members to their krewes, it seems like both are products of the same process with very interesting intent.
Here we stray into hypothetical territory.
There are no Tier 3 krewes in existence, and it’s all but impossible to find evidence of their existence. I personally believe that once a krewe grows powerful enough, they descend into the Underworld and birth a new Dead Domain, with at least one of the contributing Geists as the new Kerberoi.
If that’s true, then we have two seemingly unrelated kinds of creatures born from the remains of something once living, bound to intelligent hosts and hoping to claim a position of relevance tied to death as a whole.