The Completionist cape is the most powerful cape in the game of RuneScape. When purchased, it comes along with a completionist hood. It can be bought in Varrock Museum for 5,000,000 coins from the Museum guard on the top floor by members who have all skills trained up to 99, and Dungeoneering up to 120, completed all quests, tasks, and miniquests, and unlocked all the music tracks, prayers, and magic spells. The cape can then be upgraded to a trimmed version after completing a list of requirements including 5000 Castle Wars games and a full statue of Dahmaroc in the player's house.
As new content (quests, tasks, accomplishments, etc.) is introduced to RuneScape, a lot of it ends up being added to the requirements for the regular/trimmed completionist cape, which makes it extremely hard to wear for a long period of time.
The cape dates back to the Second Age, when it was enchanted by the mage Dahmaroc.
Spirit cape: Option to toggle the ability to use 20% less summoning special energy when using scrolls. You must use a Spirit cape on the Completionist cape to unlock this function.
Members can buy the cape and hood on the top floor of the Varrock Museum for 5,000,000 coins from the Museum guard. Anyone without the requirements to touch it will be turned into a random animal (including monkeys, squirrels and chickens) and knocked away from the cape. They will also be given a list of remaining requirements.
To wear the trimmed Completionist Cape you will need the following in addition to the previous requirements. Note that investigating the empty cape rack in the Varrock museum will provide a list of things still to be done.
Like Clan Cloaks and the Max Cape, the colours of the Completionist cape can be customised by using the the option in the right-click menu of the cape.
Similar to the Dungeoneering master cape, this cape leaves a white particle trail behind the owner. These particles change to gold when upgraded to the trimmed version. The particle colour is not customisable.
Players who have managed to achieve this cape can perform an emote in which they transform into a dragon, fly up and hover in the air while burning a classic "RS" sign into the ground before landing and transforming back into a player. The RS symbol disappears when the emote is over.
The dragon in the trimmed cape's emote is gilded, like the trimmed cape itself.
If this emote is performed with a trimmed cape, your dragon form will be equipped with a set of golden dragon armour and your wings will have their webs gold-plated as well. The movements of the emote however, will remain the same.
The Completionist cape beats every other cape in all fields of equipment bonuses — except for Magic attack, in which the god capes give the same bonus, and weight, which is higher than several other capes, including spotted and spottier capes. However, it is only on par with the TokHaar-Kal in defence and strength bonuses.
After the player buys their first completionist cape, their Adventurer's Log reads: Bought my first completionist cape. I visited Varrock Museum and took rightful ownership of my completionist cape.
After obtaining the trimmed cape, their Log is updated with: Upgraded my completionist cape. After incredible effort, I unlocked the final enchantment of Dahmaroc for my completionist cape.
A trimmed completionist cape is still just called "Completionist cape" as opposed to "Completionist cape (t)".
The message when someone anywhere gets a Completionist Cape. Achieving the Completionist cape is a message that gets broadcast to all the players in every world.
Completionist cape getting removed due to a new requirement. When new requirements get added to the cape, it will be placed in your inventory or bank.
Edit
Attack bonus 
Defence bonus 
Damage absorption 
