"You're visiting to see my gardens! My heavens, young man. You're a naughty one. You're here to talk with General von Bremer, of course. You'll be wanting to convince him that he'd be better off taking the coin of that horrid pretender you serve. You thought I wouldn't know?
But I'm boring you. You're fast asleep. Just like my third husband, Lord Meched. He nodded off during tea as well. Duke Bjarrnae, my seventh husband, he was the same way. Or was he my ninth?
Oh, where has my memory gone? It must be all the arsenic. Or was it the belladonna?"
Two types of agents serve you in your struggle to unite Lyssan: Armies and infiltrators. Knights and nobles command armies. They fight each other, control territory, and keep the local gentry paying their taxes to the one true Emperor of Lyssan. That would be you. Spies and priests are infiltrators. They do their work in territory your rivals control. Armies can't harm infiltrators and infiltrators can't hurt armies.
Unless you have the right surprise ready.
Old Age and Treachery lets a noble bump off a spy or priest. It's an archetypical surprise card. Spies and priests glide past bumbling nobles day in and day out, until one day they step into the court of a lord who knows a bit more than they let on. A nosy spy or an older brother in line for the throne. Either can be invited out to hunting party that ends in tragedy. A relative who entered the priesthood to sidestep the family politics, or one who's tampering in affairs for a distant lord's sake. Either could expire after a brief but convincing illness, with the help of a sympathetic apothecary. And the advantage of sitting on the throne is that no one asks the inconvenient questions afterwards.