Immortality (不死, Fushi) is a special ability that renders an individual unable to be killed (by regular means). This ability is notably carried by the immortal clan[1] and death gods.[2]
Overview
Immortality allows for an individual to be long-lived (longevity) and retain a level of youth even when aging and also allows for potent regeneration abilities.[1]
Immortal Clan's Immortality
A clan made up of werewolves, those in the immortal clan possess a form of immortality that allows them to survive even the most grievous of wounds and regrow limbs. However, their immortality has limits; permanent injury and even death can be occurred by Witch-Hunt and its various power-up attacks.[3] In the non-canonical video game Soul Eater: Monotone Princess, a immortal can lose their immortality by consuming a kishin egg.[4]
Death God's Immortality
As Gods, the Death Gods are immortal, unless they are Immature such as Death the Kid before connecting the Lines of Sanzu, making them impossible to kill by normal means but even they can be killed under a specific circumstance.
Whenever a new True Death God appears, the current True Death God will die as the world doesn't require there to be more than one, as Death died as soon as his son, Death the Kid, connected all the Lines of Sanzu and become the new full-fledged Death God, taking his father's place.
In the anime, when Kid connects the Lines of Sanzu against Asura, a crack appears in his father's mask, but he doesn't die due to Kid having only connected them temporarily, so it wouldn't kill him unless the connection was permanent.