An Ochema is an avatar of an archmaster that has been sent back into the Fallen World. The Exarchs use them iconically as means to communicate with their servants in the Fallen World. Seer doctrine says that ochemata are “sheaths” or “shells” cast off by the Exarch: a fraction of his true power.
Ochemata descend from the stolen Supernal Realms to assassinate gods and Invisible Masters, kill cabals with mighty mystic secrets, or inundate rebellious nations with disasters: fire, flood and pestilence, although the Pax Arcana hinders them from acting in a too blatant way. Of course, sometimes the Exarchs cheat and their Ochemata have to possess mortals in order to do their bidding.