Based on the isekai novels/manwhas I love so much.
I might make more, branch out to x servant reader, just prince, depending on how people take this.
NOBILITY RANKING
In this story, The L/N family has a viscount role: In the Carolingian period of European history, the vicecomites, or missi comitis, were deputies, vicars, or lieutenants of the counts, whose official powers they exercised by delegation. (Britannica)
They are still allowed to marry above their ranks, though with less power it's not encouraged. In short, they are almost the bottom of the chain, which your yandere can take advantage of.