Lord Dramath says that Tenebrae lured the other sithlords into a dark ritual that consumed them and all of the live on Meedrias and turned it into Nathema.

Zildrog says that he hasn't felt this hungry after devouring this planet (Nathema).

So who actually devoured the planet? Tenebrae or Zildrog?

Or did Tenebrae make a pact with Zildrog, luring in the sithlords of Meedrias into the seats, starting the dark ritual leading to Zildrog devouring all live force of the planet and sharing this power with Tenebrae, upgrading him into a god.

Zildrog has the power to somehow upgrade people and droids alike. Tenebrae/Vitiate/Valkorion can also share his force power with loyal subjects, so there must be a connection between them.

The two are not mutually exclusive

The Sith Emperor's ritual always required a mass death to be triggered by an outside source - he plotted to initiate a war between his Sith Empire and the Galactic Republic. The deaths caused by the conflict would fuel a new ritual that would allow him to consume almost everything in the galaxy and become an all-powerful god. But Revan and the Treaty of Coruscant slowed him down. After freed from Sel-Makor's grasp the Emperor attempted to initiate his galaxy-wide ritual again, wars, the atempt to blow up Belsavis, etc - before Vitiate could use the life essences of the dead to empower himself.

As per the Revan Novel:

Every man, woman, and child on Nathema died that day. Every beast, bird, and fish; all the insects and plants; every living being touched by the Force was consumed. When the ritual ended, Nathema was no longer a world. It was a husk sucked dry. Lord Vitiate sacrificed millions, stealing their life force to make himself immortal."

―Darth Nyriss

The Zildrog Codex entry says:

To achieve his goal, Vitiate used the central computer of Zildrog, an ancient superweapon from the artificial world of Iokath that specialized in the transfer of life energies, to empower himself at the cost of consuming the life force of every living thing on the plane

So, all and all, Zildrog was the catalist of the death that fueled the ritual power, it indeed consumed everything, and then with sith sorcery and technology transfered the energies in to Vitiate