He has the artefact.

"Well, as long as you explode closer to our enemies than to me, I can't really complain."

"I can handle that."

... this has been a night. Too much of a night. "I will stay."

...right, he's carrying the artifact. 

"...if Shadowheart and Lae'zel want Gale to leave we can make Wyll carry the artifact. Sorry Wyll."

"What artifact is this...?"

"Gale might explode. Wyll has a patron with her claws in him. I think it should stay with Gale."

Why does Shadowheart have to be so reasonable? "I agree."

"...Good point." To Wyll: "there's a githyanki artifact that Lae'zel and Shadowheart dispute ownership of. Gale is carrying it for now and they'll fight for it once we're all mindflayerless."

"I understand." No,  he doesn't, but it's so beyond his circle of concern right now that it doesn't matter.

Cayden eats his barely-edible camp food, thinks about the artifact, and doesn't make eye contact with Shadowheart. 

He feels like a pile of shit. Why does he feel like a pile of shit? ...Because he's planning to betray her and she doesn't know and thinks that he's trustworthy.

Okay, when you put it like that.

After dinner, during which he's mostly silent, he goes to Shadowheart and says, "can we talk in private?"

She arches an eyebrow. "You seem to be talking to a lot of people privately. Sure."

"A lot of people have things they don't want to say publicly!" he says, walking farther from the camp than he did even with Astarion.

"We all have our secrets. I'm not sure whether I'm more surprised by Wyll's or Gale's."

He's not saying anything about them. 

"...I just want you to know that when you and Lae'zel end up fighting over the artifact, I'm going to be fighting on Lae'zel's side."

"And you thought to tell me this why?"

"You shared something with me and-- it's not fair for you to go through the adventure thinking I'm on your side."

"I understand." She's trapped. Lae'zel, she could take, but both of them at once? Following the will of the Goddess may be beyond her. "You know, this was a foolish thing to do? Your only protection at this moment is the fact everyone would assume I had killed you.  Are you really going to trust your life with that?"

Wow, does she have another thing comi--

Wait. Energy drain. Shadowheart could maybe take him. 

Oops.

He spreads his hands. "They call me lucky, not smart."

"And you're lucky I'm smart enough to not to do something stupid. Still. No one could hear you cry for help, and I'm pretty sure I could convince some of them you had started changing."

"...I think we have Shar on my homeworld." And he's running the most amazing scam claiming to be one of the oldest gods in the universe. "Individual worshippers of Shar are often great people. Some of them are my friends. i'm not even against the groups that are just a religious spin on organized crime. But the real cults-- the real cults are bad. They destroy people's lives, worshippers and victims both. I-- still have nightmares about some of the things I've seen. If I knew you just wanted to sell the artifact to the highest bidder, I wouldn't care. But I don't know anything about what they want it for, other than that they want it badly enough to erase all your memories-- that scares me." 

How is she meant to respond to that? Her cell quite plausibly does the things he dislikes, she wouldn't know. And she doesn't know why they want the artifact. "Someone else could be worried about what the Gith will do with it. But I know I'm not going to convince you with that."

"You could. There are a lot of groups worse than cults of Shar."