After findign out that he's immortal, Nicolo (perhaps after escaping the Church that wishes to experiment on him) seeks refuge with the enemy. Yusuf says he may stay within the city walls if he becomes Yusuf's bed slave - which he proposes only to humiliate him, thinking the religious man will surely walk away from such a deal. But Nicolo promptly accepts, as long as Yusuf vows never to leave a mark on his skin, thus exposing his secret.

And now they're both stuck with a bad arrangement.

Consensual with both characters being equally reluctant, at east at first.