The source is always magic, but in the case of vampires and immortals I think that what gives more strength to their immortality are the deaths caused to feed, I think the gods would also be considered, in season 4 it is said that the gods punish humans with catastrophes: earthquakes and fires? they said that believing in them keeps them strong, I had a theory, fearing them can also give strength, it's a form of belief too, having fear makes people believe, but I think the death of these various people in these catastrophes also feeds Immortality ( The Hollow used it to increase their dark magic, and Silas used it to maintain their psychic powers), so in a way immortals are freaks because of that detail? are they surrounded by death whether it's feeding: vampires, immortals and sirens? or causing catastrophes: the gods? I think Landon might be a different case too, I think his own death fuels immortality, (in the sense of dark energy/magic caused by death as it was for the Hollow? only in his case it's death itself that gives him energy to return from the spirit world).

I see yeah perhaps

Malivore one I want people forget me

Huh?

Malivorie place people forget you

Yeah that has nothing to do with my post though

You put malvoire on your post

The Necromancer immortality I believe comes from the magic transforming him into a being who has his foot on both planes. Similar to what Bonnie was when she was made the anchor.

Malivore immortality should have come from the fact he was a being made of mud. Basically he was born as a being without a real life-span to begin with. Plus, the beings he absorbed probably helped prolong the magic that helped kept him animated.

Phoenix may have been similar to Malivore. In the sense that it may have never original been a living being to begin with.

As for the Gods, Jen already told us their immortality comes from those that believe in them. 😎

Cool

Cade and Inadu's immortality was probably due to the amount of people they [their followers] killed over time as showed that the Sirens killed people to be immortal due to the accord they did with the devil and Inadu's followers used her bones to use Sacrificial Magic to help her to escape from the Ancestors' control. Also, Cade only became immortal after dying, which means he probably used those poor (or nor that poor) souls to increase his powers and turn himself into an immortal. Inadu's bones was being feed up with magic for years as I said, so that's probably the reason why.

Necromancer's powers never were explained properly in special, but could he use his own powers in himself to bring himself back to life? He could still use his powers even he was dead or trap into Malivore and in the Limbo, so It is a suggestion, but probably no. So I'm guessing his powers was due to his nature: since he can manipulate and control death, he has a certain invulnerability over death itself as he showed he could only use his power if he becomes "evil".

Malivore's case was probably due to the people he absorbed. The unity of a witch, vampire and werewolf's blood along to the Dark Magic created a living dimension inside him turning him into an immortal being due to absorbing so much energy inside only one creature. It is similar to Inadu's case. Her bones gained so much energy that due to the sacrifices that her own body created a kind of immortality to prevent herself of dying.

Landon's case is weird. We know he had the Phoenix' powers and we know what a Phoenix do, but if you're not sure: Phoenix is born form its ashes. Translating: Landon probably uses his own dead as a conduit, so making him pay the price and bring himself back to life at the same time. We didn't know how many times he can die only he had a weakness like all immortals.

About the gods, I already said something about it once and my opinion still remains: Something stronger than the gods gave those powers for them, making them into immortals and powerful being to protect something, someone or from something, someone.

About Dahlia, we already know where her immortality comes from: Connective Magic. It is still a mystery the reason why Nature did allow Dahlia to become immortal and still keep her magic due to the amount of magic she had and even she woke up for just 1 year, she could cause a lot of chaos using that magic and her immortal body to channel the quantity of energy of her body she needed. I mean, her firstborn magic was already insane, so the mystery is not how she became immortal but why Nature allowed her to be such thing when all immortals had their magic cutt off (heretics and Hope are exception).

Of course, just like everything in here, it is just my opinion and a guess nothing of this is a fact.