A Lust Imp in disguise

Playing the long game would be the best option. Yes, the demons could theoretically charge in guns blazing, as it were, but the chances of the Angels taking notice and immediately smiting the invaders would be at their highest. No, the Demon Lords needed to wait and slowly infiltrate the monastery with one of their own in disguise.

Ulexes, The Unsuspected was called in for the job. He excelled in fulfilling a niche not many other demons had, even those capable of assuming a human disguise: looking vulnerable. Whenever he disguised himself, he looked like the kind of barely-legal young adult that preferred reading books and learning magic over training endlessly and taking up a weapon to fight in blood-soaked battlefields. He was 5'4, had wavy, messy light brown hair and slightly bigger eyes. To help look even more vulnerable, he also materialized a pair of glasses onto his face and managed to make freckles appear on his permanently rosy cheeks.

But a Lust Demon remained a Lust Demon, and with enough careful manipulation... He would have the first nun addicted to his dick in little time.