Can anybody think of any mechanics to represent Ax'senaea's the Thrice-Possessed tricks regarding eating daemons (who are possessing her) for breakfast? I'd be interested in trying out something similar.

If it's something you want to do in an existing game, you'll need to talk to your GM.

As a starting point, the most obvious thing to do would be to use a version of the rules for creating Daemon Weapons, or some variation on the Demonic Mastery Test.

If I was going to GM something like this, I'd probably require a series of rolls to represent the battle between the character and the Daemon, probably over an extended period of playtime.

It could be the series of daemonic mastery rolls till either you or daemon accumulate set number of DoS over the other. Such battle of wills could span for weeks, months even if both combatants WP is high enough.

A nastier way to run it would be to require the character to straight up *kill* the possessing entity through damage dealt by resisting possession attempts.

So once you've rolled to resist Possession, instead of letting the creature go, you could instead opt to make a Willpower roll to try to trap it, resulting in the creature getting another shot at your body, but in your gaining the chance to whittle it down by force of will (a really hard version of this would be to require you to kill the creature before it succeeded in a *single possession roll* but that would I suspect be borderline impossible).