Oneness
When HoT fought Tenebrae he wasn't fresh, he'd just fought through hordes of powerful Sith.
But Tenebrae had been dispelling all the accumulated life essence he'd absorbed in his ritual on his subjects for a thousand years.
Notice how the expedience in which he'd been able to eliminate each new Dark Council he created waned over the years. His first Dark Council was the strongest, his second filled with less talented Sith so he'd more easily control them. He eliminated his first Dark Council "in a flash of light".
Then it took him quite some time to eliminate later dark councils, and he could have been destroyed in his battle against Revan and Meetra - which he still won without a lightsaber.
Now against HoT he'd been fighting with a lightsaber, relying heavily on illusions.
IDK, but you don't lose to a any number of Jedi, however powerful they may be, I don't even care if it's Yoda + Starkiller II + Exar Kun + Ep 7 Luke Skywalker, when you possess trillions of times a normal life force - as Nihilus and Tenebrae would have possessed as fully fed nexuses. Palpatine, not quite there yet.
Nihilus consumed a world all at once, and his powers decreased as well, as he was starving, before facing Meetra and Visas, wounding the latter insured his defeat in that instance as well.
Tenebrae and Palpatine both ate the life essences of planets over a longer period of time, Tenebrae feeding upon more life forms and fully consuming them, and so yes Tenebrae's power was more sustained and was used up more slowly. But it was being used up none-the-less. He wasn't utterly immortal, as Plagueis himself claimed that Tenebrae was closest to achieving immortality, not that he had become entirely immortal yet.
But Tenebrae had been dispelling all the accumulated life essence he'd absorbed in his ritual on his subjects for a thousand years.
Notice how the expedience in which he'd been able to eliminate each new Dark Council he created waned over the years. His first Dark Council was the strongest, his second filled with less talented Sith so he'd more easily control them. He eliminated his first Dark Council "in a flash of light".
Then it took him quite some time to eliminate later dark councils, and he could have been destroyed in his battle against Revan and Meetra - which he still won without a lightsaber.
Now against HoT he'd been fighting with a lightsaber, relying heavily on illusions.
IDK, but you don't lose to a any number of Jedi, however powerful they may be, I don't even care if it's Yoda + Starkiller II + Exar Kun + Ep 7 Luke Skywalker, when you possess trillions of times a normal life force - as Nihilus and Tenebrae would have possessed as fully fed nexuses. Palpatine, not quite there yet.
Nihilus consumed a world all at once, and his powers decreased as well, as he was starving, before facing Meetra and Visas, wounding the latter insured his defeat in that instance as well.
Tenebrae and Palpatine both ate the life essences of planets over a longer period of time, Tenebrae feeding upon more life forms and fully consuming them, and so yes Tenebrae's power was more sustained and was used up more slowly. But it was being used up none-the-less. He wasn't utterly immortal, as Plagueis himself claimed that Tenebrae was closest to achieving immortality, not that he had become entirely immortal yet.