Assassin Armor is a gear perk that greatly cuts the noise you make while moving, but lowers your general defense. It is a trade-off built for pure stealth play.
Guards hear you much less, so you can walk closer and faster without being noticed. If a fight breaks out, you will go down quicker than usual.
Wear one of the gear pieces that carry it: Altaïr's Robes, Fry Cook Slacks, Posh Jacket. Each piece only takes one body slot, so plan around the other perks you want.
Great for players who rarely get spotted. If you often end up in firefights, the defense loss is a real risk.