Chronomancers (which is an Onirian term for them, also known as the Lost Peoples (often derogatively, by Umichooans) are a race of nonhuman beings that come from somewhere or somewhen else, tend to take human forms to interact with the locals in our reality, and seem to have a control of the flow of time way beyond what any local civilization can manage.

The first Chronomancers were spotted some centuries ago in the Onirian time branch, but given both the nature of the Veil and their skill at passing by regular humans, they might have been around for a lot more time.

Assuming they are just not time-traveling to the centuries they like, because apparently they can do that.