What's the noun for betrothal? Here's the word you're looking for.
betrothal
The act of betrothing, or the fact of being betrothed; a mutual promise, engagement, or contract for a future marriage between the persons betrothed; betrothment; affiance.
“The same day that I received from Zurich all the papers of my dear betrothed, and all the relics of my departed love, which I hastened to enclose in the black casket.”