Ask him help, he will talk about mordred and errant knights, ask him errant, he will tell you that mordred is trying to recruit them and to bring him any information you find. Find an errant knight. Ask them mordred. They will attack you, they may spawn a map, get map corpse, give it to the earl, there are 4 possible outcomes from giving him the map:
-one is a chest in the Cambrian mountains (At an Abandoned Camp (mispelled in online map)) chest has 3 marks of chiv, key is on corpse of reneagde soldier at same loc
-Another option he will tell you to go kill Mychal. Mychal spawns at the Ambrosius dyke
-Another option is to kill the renegade leader in Cornwall (A Road in Cornwall), after killing him you get a buff to find more marks in general
-lastly you can just get xp