KILL SIX BILLION DEMONS: Beginnings are false and I am a consummate liar.

[Aquillion]

I'm not at all convinced that it is true. The other 6 demiurges certainly don't seem to have trouble keeping themselves alive, and there's no way Mottom would be giving Jagganoth the fruit, in particular (not to mention Zoss sustained himself long before Mottom was even a demiurge)

As I recall, the author has said that each of the demiurges has found their own way to extend their lives. The keys don't grant it naturally.

Going over them...

Jagganoth probably has immortality as part of whatever he did to himself with those angel feathers.
Jadis is clearly in some form of stasis.
Mammon is a Servant; his race doesn't die of old age anyway.
Gog-Agog is a composite entity, being a collection of worms. I assume that some of the worms die and others are born or added.
We know Mottom uses her fruit.

That just leaves Incubus and Solomon with unexplained methods of preserving themselves. Presumably they've each found some method that they either refuse to share with Jadis, or which wouldn't work for her for one reason or another.


Also, more from the author's tumblr regarding power levels:

Some are definitely more powerful than others (in personal abilities). Probably the rankings I’d have to go with are Jagganoth > Jadis (pre-entombing) > Solomon >Gog-Agog > Incubus > Mottom > Jadis (post-entombing) > Mammon.

Mammon only for reasons that will become apparent in the next book. He’s a monster - he’s called the Grand Dragon for a reason.

No angel is more powerful than a demiurge by the way save Metatron - I’ve seen people mention that and not sure where they are getting that from.

The last bit seems to people doing the same math I did above with regards to "Jagganoth = 1 Goku, Juggernaut Star = 0.75 Gokus, all the other Demiurges combined = Jagganoth."

Presumably power levels don't stack together, so the other Demiurges are somewhere between 1 and 0.75 Gokus, but don't just get to add all their power levels together to fight Jagganoth (eg. Goku can beat up an endless parade of people at 0.8 Gokus.)