The handwritten message is an item found in the Tomb of the Fallen during The Death of Chivalry quest. It shows Lensig tricking Black Knight soldiers into fighting against you and Sir Owen.
Transcript
Lieutenant, Gather your best men. Two knights of Saradomin will soon reach the tomb. Tell no one of this. Protect the prisoners with your lives.
And be careful when you pass through the portal. I don't want to have to pay that witch to re-cast the ritual.
Strength through chaos,
Captain Dulcin