Try to talk with the possessed adventurer first

Then again... The adventurer would know her own abilities. Which could be very useful to you considering the fact that you are about to fight with adventurers, if you don't use all your capacities they might get suspicious.

They are also going to get suspicious if you start talking to yourself though. You need an excuse to get away while you speak to the adventurer. How can you get them to go away for a little while?

Then you remember that almost no adventurer ever notice the secondary questline of the haunted mansion, even though the reward weapon is pretty nice for a low level one. And it can actually be levelled up if you follow the Heirloom questline.

And all they had to do to activate it was read the diary in the decrepit bedroom, it would tell them all about the weapon on how to obtain it, it would even hint at it's hidden potential. But adventurers never stopped to appreciate all the little details of game design. Idiots.

"Hey, before we go further. Did you know that there is a secondary questline in this tutorial area?"

That obviously surprised them. Typical. "What? There is another quest beside the 'kill the monsters plaguing the ruins' one?"

"Yeah, there is a book in the bedroom, when you read it it activate the 'give rest and clease' quest, you need to get the fertility idol and place it on an altar after the boss and the spirit of the owner can rest, it gives a unique weapon as a reward."

They look at you like you've grown a second head. "O. M. G. That's what the fertility idol in this area is for? I thought that was random loot, worth a few gold pieces at most... Thanks. We'll go read that before going then."

"Okay, I'll wait for you there." That should buy you some time to talk, and you'll still be able to get the weapon. Another thing adventurers generally don't know: you don't need to activate a quest to complete it.

You wait until they are out of the room before saying, in a low voice. "Okay, now we can talk, but if you try to scream or alert anyone I'm taking control back."

You religuish your control over the adventurer's vocal cords and mouth and she sigh, resigned, by now she's probably realized that she wont be getting out of this any time soon: "What do you want?"

Taking control back is as easy as you expected: "To live. Look I'm not trying to take over, I'm sure we can find a way to make this mutually beneficial."

Her laugh startle you, you weren't expecting that answer. "Yeah, can see what kind of benefits you have in mind. But what's in it for me? Not that I have much of a choice..."

"Well this is is new for me too so there might be ways to escape that I'm not aware of. I'd rather avoid that, your body feels great, I don't want to leave it. On the other hand I don't have to possess you all the time, I could just use it while you're AFK."

She takes her time to answer and then sigh. "Okay, I guess it's better than nothing. We have an understanding Anshee. By the way, I'm Vana Silverwind."

You hear noise from the next room, the adventurers are coming back. Quickly Vana explains her abilities and you take back your control over her body.