The Missionary Achievement
Collect all Relics in the first El Cid mission.There are a total of five Relics on the map, four of which are located in a clearing in the far east of the map below Alfonso's base, and Alfonso has the final one. Once you have a base you will want to take some men to the far east and gain control of the monks there along with the Relics.
Once you have these, start attacking Alfonso's base. His monastery is located to the north of his base.
Note: If he changes stance and becomes allied with you before you can destroy it and take the relic just use a Mangonel on attack ground.