Zansei

Zansei (残血清 Lingering Serum): A unique serum created by Muramasa. Though it's effect is noted to be more magical than scientific, the serum itself is noted to be a pinnacle of scientific medicine

Overview
According to Muramasa, the serum itself is made of a liquid which is extremely adaptable. When injected into an individual, it spreads through the vody via the bloodstream, and adapts to each and every cell in the body, storing itself until needed. When the body is about to die out, it produces a unique "regenerative" effect, classified as such only due to lack of a better word. Trough a massive burst of reiatsu from the hands, feet, and head, the user's entire body is altered into that of a new appearance, though the age itself is random, younger than before is usually common. Enhanced abilities, also random, are noted. For example, one could have enhanced speed or strength. Despite the miracle such a serum is, notable side effects include brief euphoria and a slightly different personality with it's variious cravings that kick in soon after gaining the new body.