General Exalted Discussion Thread IX: Errata is Combo-Ok!

[Drascin]

We know how mote-draining charms work; there is just no reason why you should read it other way in onieromancies. It is not as possible to store uncapped quantity motes in simple artifact.

That is precisely how most people read it in my experience, though. The whole "you have to go and get the stored motes by touching it" bit clearly implies that, unlike all mote-drainers, the motes aren't instantly put into your essence pool, but rather stored in the artifact itself. And it doesn't mention a capacity maximum anywhere.

Most STs, however, put a cap on how many motes the oneiromancies can hold, of course.