Domains: Ritual purity, ablution, the cleanly-prepared.
Era of ascension: Golden Age of Man (~170,000 years ago). Cohort: dead. Died: God War & Long Winter (~75,000 years ago), when the rivers froze, no ablution could be performed, the rites that ablution preceded could not begin, and her cult collapsed almost passively as the worship of other gods stopped requiring its own prerequisite.
Worshipped by: Priesthoods across the Golden Age cults — purity preceded most rites, and most rites were not Cessaréth's — and the institutional bath-houses of the Golden Age cities, which were her temples in everything but name. No living culture maintains her worship. (The worships edge is authored downstream on the worshipper, not here.)
Cessaréth was religious infrastructure. Her worship was a prerequisite to other worship: before a priest of any major Golden-Age cult could approach the rite, they passed through a Cessaréth ablution: a structured bathing, an interval of silence, a re-clothing in unmarked linen. Her doctrine cared only for the cleanly-prepared state of the rite-performer at the moment of crossing into the other cult's sanctuary, not for the rite that followed. Her priesthoods were therefore everywhere and almost invisible, the bath-house at the back of every temple complex was hers, the bath-keepers were her clergy, and her name was spoken briefly at the start of every formal rite by people who would then go on to address other gods for the next hour.
The Winter killed her by a cascade. The rivers froze (see Onnoróth); without flowing water there was no ablution; without ablution the priests of the major cults were not in the prepared state their rites required; without the prepared state the rites could not begin, or began deconsecrated and were known to have been deconsecrated, which the cults could not theologically tolerate for long. Within a few years the bath-houses had emptied and her clergy were dispersed; within a generation, the major cults were performing reduced rites that simply omitted the prerequisite step, on the grounds that no preparation was less wrong than a false one. Her absence was not noticed individually, which was the structure of her worship, but her absence was the door through which the cascade collapse of every other prerequisite-dependent rite walked.
Sorveth is the symbolic clarity of daylight; Cessaréth was the bodily clarity of being washed. The Winter denied the body its rite and the symbol followed soon after.