I have also spent quite a bit of effort to achieve this one, however it not not trigger in steam.
Heartbreaker
Marry the widow of a lord or lady you personally executed.
I have hunted down the current wife of one of the vassals, had her executed, married the widower, and nothing happended. I have saved the game immediately before I killed the wife, and repeated the process several times.
No mods, no cheats.