This ordinary looking brown cloak is strangely tough and coarse and faintly radiates magic. Without being soaked in blood it's true potential will never be known.
However, when soaked in blood it gains great resistance to damage and acts as an armour for the wearer. With every hit the wearer takes, blood from the cloak is lost (splattered everywhere) and slowly the cloak loses it's own blood supply and also it's ablility to act as armour. The cloak soaked in blood is magically always dripping (making the wearer easy to track) and fresh (the cloak is always bright red when fully loaded - no coagulation here). The wearer of a fully loaded blood cloak tends to attract vampires and other blood drinkers. The wearer will obviously be sent away from fine establishments and people who value their furniture. He/She will constantly be directed to the local hospital/healer/temple by concerned people. Adventurers who depend on the cloak will remember to soak it in the blood of their kills (which is the best way of recharging it's power). Obviously it takes a certain kind of person to wear this cloak, I'm not making any judgements.
The Mechanics of the Blood Cloak
The cloak has a certain number of hit points when fully loaded. The cloak acts as damage resistance absorbing the first bit of damage and taking it off of it's own hitpoints and the rest coming off of the wearer. Once the cloak's hit points are gone it is out of blood. When the wearer takes damage from a piercing, slashing, or hacking weapons the number of hitpoints the wearer loses are added to the cloak's hitpoints as long as that does not take the cloak above it's maximum (providing the wearer does have blood sorry warforged etc.).
A cursed version of this item might keep it's wearers wounds open (preventing natural healing) and contiue drinking the wearer's blood until they are magically healed or the curse is removed.
Well, that's my first submission. I hope it makes sense and is somewhat enjoyable.