Radcliffe sews up many of the mysteries by the story's end, leaving us with one big question mark about Emily and Valancourt. And we get a funny word to end the book: Em and Valancourt are "restored" to each other, to their properties, and "to the securest felicity of this life, that of aspiring to moral and laboring for intellectual improvement" (4.18.8).
Okay, so this is a story about two people losing everything and finally gaining it back. We always knew they were worthy folks, so that comes as no surprise. The next step, apparently, is for both Em and Valancourt to constantly improve themselves intellectually and morally.
And then it sounds like Agnes the fire-and-brimstone nun takes over the pen:
Though the vicious can sometimes pour affliction upon the good, their power is transient and their punishment certain […]. (4.18.10)
The narrator wants us to know that though things might seems a little ambiguous, the evil will always get their just desserts. Namely, a cup of poison.