The Cape can be obtained by purchasing an Achievement Diary Hood from Twiggy O'Korn located in Draynor Village which is located to RuneScape gold the south of bank. He sells it for 99k GP only for players who have had all Diaries completed. It is, without doubt, among the most difficult items to obtain in the game because it requires not only very high skill levels but also the achievement of various tasks which often include lengthy quests.
Achievement Diary Cape is cut following the completion of all quests. This makes the version that is trimmed even more difficult to find. Similar to Quest Point Cape, this item loses its trimming whenever a new quest comes out. It only gets cut when all of the quests available in the game are finished. So, every quest must be completed, as well.
The untrimmed and trimmed versions of this cape offer +9 bonus for all defensive stats and +4 bonus to Prayer capability, making it the most effective slot-less capes for Prayer. Achievement Diary Cape also has 10-day teleports to anyone who is an Achievement Diary Master, which makes it an ideal accessory for transportation.
However, some of the achievements are located during Draynor Village, Varrock, Lumbridge and other open to play locations, Achievement Diaries are not accessible to buy RS gold non-member players. Therefore even doing correct actions will not give the respective benefits to the player playing f2p. While there have been many reports about the possibility of creating new Diaries exclusive to free players, there is no proof from Jagex.