I don't think I ever mentioned this anywhere, and the comic doesn't really address it, but that's Vae getting sucked off in this comic. She used magic to give herself a dick for the purposes of Isaac's training, because she's a really supportive and selfless partner like that.
Someone on cohost left a comment asking about the reward that Caelle mentions. I am happy to report that Isaac's reward for being such a good cocksucker was the privelege of doing whatever he wanted with Caelle and Vae for the rest of the day: