Cephaliarch is not a common word in English. It could be a combination of cephalic, which means relating to the head, and arch, which means chief or leader. So cephaliarch could mean head of a group or organization.
When people say to me........
"You're going to regret that in the morning"
I sleep until midday cos I'm a problem solver!
Assuming that it's just a made up word but maybe based on Latin/Greek:
"Cephali" is a literal head (body part) from L. cephal- / A.Gk. kephale; it can also denote something which is figuratively or literally first or superior: the head waiter, head of the class, a heading; it can be also used in different forms such as "encephal-" to mean "brain."
"Arch" as a prefix or suffix is used to denote possession, ownership, leadership, or being a ruler/having power over something or someone: archbishop, archmage, exarch, hierarch, archimandrite
One possible very free translation might be "Ruler of Knowledge" which would fit Hermaeus Mora's title as the "Abyssal Cephaliarch" quite nicely.
Paralyse, Sanguine's Tester - Enjoying ESO since beta. Trial clears: vSS HM, Crag HM's, vRG Oax HM, vMoL DD, vKA HM, vCR+1, vAS IR, vDSR, vSE