Assassin is a gear perk that makes you move faster while crouched. It is a core stealth perk for getting behind guards quickly without standing up.
Sneaking speed goes up, so you can close the gap on a guard or slip between cover without breaking stealth.
Wear one of the gear pieces that carry it: Dark Tech Boots, MK VII Tac Boots, Posh Shoes. Each piece only takes one body slot, so plan around the other perks you want.
Yes for any stealth build. It is one of the most useful boot perks for clearing bases quietly.
Pairs well with Chameleon and a quiet Amigo like Chorizo or Oluso.