Durnehviir is a dragon that inhabits the Soul Cairn. You can learn one word of the Soul Tear shout from him each time that you summon him with his shout. He cannot be truly killed, as he will reconstitute himself in the Soul Cairn upon being sufficiently injured.

Should I summon Durnehviir?

Durnehviir, as the shout name suggests, is considered a summoned creature, and will be replaced if you summon anything else, unless you possess the Twin Souls perk. It is possible to maintain Durnehviir and Odahviing at the same time. You must spend three dragon souls to fully learn this shout in order to summon him.

Does Durnehviir go away?

Durnehviir acts like a conjuration spell so after so long of being around he will vanish on his own. Takes about 3-5 minutes I think before he does maybe less but he does disappear. ... Durnehviir acts like a conjuration spell so after so long of being around he will vanish on his own.