Note that we don't get traits USEFUL in things we've done.
We get traits making it easier to replicate some of our achievements in the arc.
So, examining vagaries of possession, what is possession? We have a Priest of Gazul on a nat 100, explain it: Possession keeps your mind and body, but your soul is replaced. Its very distinct.
Avatar has nothing to do with possession. Possession was our uninformed term to explain being a conduit for divine power, and has expressedly been ruled out.
So what did we do as a conduit for divine power?
-Ranald -> Mathilde - Granted us good luck as a favored worshipper. Attracted cats. Converted and physically transfigured a hostile shrine. Attracted Wolf. Patted us on the back.
--Most of this was uncontrolled, at most we let Ranald know that his attention would be very helpful now thanks and he did the rest. Or did not do anything, we can't tell IC.
-Mork -> Mathilde - Mork filled us with Divine Power and temporarily granted Thews worthy of Conan The Barbarian as his temporary Avatar. As a side effect, we also started speaking in his voice and got really angry.
--This was entirely uncontrolled, and accidentally solicited. It has been confirmed that its impossible unless you just perfectly sync up with Mork for that instant. The hole in our soul has nothing to do with it other than Mork having left it when he barged in by force.
--We're also very unlikely to see it happen again because...
-Mathilde -> Ranald - This is the deed that got us the trait as a reward rather than a penalty. We served as a conduit for power, taking it from Mork, sending it to Ranald. The energy passed through us like we're a pipe, we did not shape divine power in either Mork or Ranald's flavors. We used our soul of Ulgu as a pipe, transferring the contents without being affected by it, allowing Ranald's far larger soul to digest it into his flavor..
--This was controlled. We did it on purpose, and with Avatar will be more able to do it again, without as much random chance involved.
-Ranald -> Mathilde - This is the reward. He used our connection as worshipper and god to give us a divine artifact, again, passing through Mathilde to manifest in her presence.
--This was mutual consent. He took the haul and gave us the cut as the middlewizard. Most gods, even the Chaos Gods, can't generally speaking, manifest divine artifacts just anywhere.
Why does this matter?
Mathilde has learned how to deliberately send warp energy through her connection to Ranald as Worshipper and God.
This means that she's somewhat more resistant to possession, when a warp critter tries to punch a hole into her soul to wear her like a sockpuppet, she now has the option to send the critter directly to Ranald to deal with, rather than the normal option of "Clench really hard and hope it gives up" or "Clench really hard and try to stab its physical manifestation".
What does it matter to her as a wizard?
If she can send and recieve energy to her god, without being a priest and directly channeling his Lore, then she could potentially learn to do two basic things that can be achieved using what she had already done:
-Ground out excess magical energy by sending it along the conduit instead of the ground. I would not recommend trying this with more difficult spells yet, because she's had a lot of practice doing this with the ground and only the one time attempt doing it with her divine conduit.
-Pour out the raw energy of Ranald into her self or her surroundings. She's not a priest with the divine lore to shape it herself, but if Ranald himself wants to do something and Mathilde is in the right place, she can open the door to let HIM do his work. Or spawn cats everywhere. He seems the type.
Not that cats everywhere is a bad idea in Eight Peaks...