... the Buddha Nature or Buddha Principle (Buddha-dhatu) is taught to be a truly real, but internally hidden, eternal potency [or entity] or immortal element within the purest depths of the mind, present in all sentient beings, for awakening and becoming a Buddha. In some Mahayana sutras it is equated with the eternal Buddhic Self, the Essence (svabhava) or Soul (atman) which is nothing less than the uncreated and deathless Buddha himself. ... The Buddha-nature is taught by the Buddha to be incorruptible, uncreated, and indestructible. It is eternal bodhi ("Awake-ness") ... and thus opens up the immanent possibility of Liberation from all suffering and impermanence.
The eternality, unshakeability and changelessness of the Buddha-nature (often referred to as "Tathagatagarbha") is described in one Sutra: " TheTathagatagarbha is not born, does not die, does not transfer [Tib], does not arise. ... it is permanent, stable and changeless."
A central aspect of the Buddha-dhatu (sometimes called the Tathagata-dhatu) is that it is utterly indestructible, invulnerable to all harm and contamination, and truly everlasting. It is the innermost, irreducible pure core within the being that cannot be eradicated or killed.
This does not violate the Buddhist concept of Anatman because, unlike the Western concept of Soul or some interpretations of the Indian Atman, Buddha-nature is not presented in the primary Buddha-nature sutras as an isolated essence of a particular individual, but rather as a single unified essence shared by all beings with the Buddha himself.