All wills must be posted publicly in the fief transfer topic of your game. In this message, you name which heir (by ID) gets what fief or amount of money (not percentage of money). This must be done before the character in question dies. No posthumous wills, please. The Herald in charge of that games transfer will save a copy of the "will" and, upon request of the heirs (ie, the player who made the will in the character of his heir) execute the will. No request for execution, no execution. We don't want to put too much burden on the Heralds.
Magnate titles have to be transferred via the existing ("choose a heir") routine. These could not be given away in a will as they, technically, belonged to ones sovereign (so, technically, did your land and authority, but kings were less able to crack down on this.)
Your fiefs and money could be left to anyone else in the game (ie, other players, married daughters, or the king. You CAN leave the king your magnate title, if you have one,)
If you die heirless, but have married daughters, you can leave it all to her. The Herald must then move you into the new character manually. This may be messy as the system purges such positions. We cannot guarantee that everything will survive this process. But that is historically accurate, as married daughters taking over the family estates was often accompanied by a lot of legal, and often some military, activity.
The Heralds will rule on the validity of wills.
Titles can only go to the principal heir (aside from titles that come with fiefs).
Ancestral fiefs, as always, can never be transferred by the player character to another. Your current character dying does not exempt anyone from this.
You can only leave money or non-ancestral fiefs to a player character.
We don't expect many people will use this. But at least it's there.