A kinjutsu - forbidden technique of shinobi, specifically of the Waterfall Village. Its most renown wielder was the Treasurer of the Akatsuki, Kakuzu.
A powerful man who, before he even received the Jiongu, fought and survived a battle against the God of Shinobi, Hashirama.
The Jiongu turns your body into threads and stitches of black, allowing it to attach hearts of powered beings, giving yourself enhanced physical prowess. Your threads and stitches can extend your limbs, grabbing with enhanced strength enough to crush steel in your hands.
Your body can extend the Jiongu into spear-like attacks, or you can use the hearts of people as weapons and allies. Your body is immortal, allowing you to survive fatal blows as long as you have an extra heart to spare.
You can only gain four extra hearts, having four extra lives in stock. You need no chakra to use any of these techniques and each heart is supercharged to be, at the very minimum, half as powerful as you. Stealing a heart steals powers.