The achievement diary cape is awarded to players who can complete all diaries at all levels of difficulty. At the time of the cape's release, this meant completing 1 beginner diary, 7 easy diaries, 7 medium diaries, 7 hard diaries and 6 elite diaries. As more diaries are released, the requirements for the cape change, just like the quest point and music cape so can trim, and be trimmed by these capes. You can buy this from Zoe after completing Zoe's Quest. The emote for the cape is the same as the quest cape emote, only with a green instead of blue symbol. As with the two previous capes, the difficulty of the cape's requirements mean it has a higher prayer bonus (+8, not +4).