Cleon
Legend
Hmmm, I didn't see that in the limited text we had. Where is it?
Well if you could simply "whack him" and be done with it, they wouldn't have needed to bother imprisoning Ythog-Nthlei would they.
A lot of the stuff I suggested is basically made up from scratch, but the flavour text basically says he's some kind of Lovecraftian Horror and so we need to give The Dark King abilities that fit.
Anyhow, checking the source material again I couldn't find any explicit mention of billions of years, but it does specify Ythog-Nthlei is a prehuman "Elder God" and implies he has a nonstandard relationship with time.
The strongest indication of Ythog-Nthlei's unbelievable age it an inscription on an obelisk that says "During the ninth rotation of our galactic cluster in this the 34321st year of our Master’s reign, we, the remaining children of the Great Lord Ythog-Nthlei do hereby register and sanctify this, our Holy Base".
That statement uses rotations of a galactic cluster to count time! Galaxies move about on timescales of billions of years. Moreover, the sentence structure suggests the "galactic cluster rotation" is less than a "year of our Master's reign", suggesting an incomprehensibly long period of time.
For example, it takes our sun takes 200-250 million years to orbit around the centre of the Milky Way galaxy, a period called a "cosmic year", so nine rotations is about 1.8-2.25 billion years.
Even if the "Master's Years" are the same as terrestrial years (which'd be odd for a creature from elsewhere), it's at least 35 millenia old.
Here's some relevant passages from the adventure:
From the diary of a doomed cleric:
“We have been attacked by an evil beyond description. It is the Elder gods, the dark ones who eat time! Only in the last few hours have I realized what is truly happening to us. Years ago this area was found to have incredibly high mana, the power on which all magic feeds. To take advantage of this natural wonder, my ancestors built this temple on top of the mana source. How were they to know that they were not the first? How were they to know that beneath our stone corridors there resides another complex, built by darkly evil inhumans in millenia past. The earthquakes must have awakened them, and now they rise again in their attempt to conquer the world for their evil lords! How are we to—Wait, a tremendous crash in the oracle chamber! Voices, too, I think. I must investigate.”
Writings on an Onyx Pillar“During the ninth rotation of our galactic cluster in this the 34321st year of our Master’s reign, we, the remaining children of the Great Lord Ythog-Nthlei do hereby register and sanctify this, our Holy Base. From these divine depths we shall build and grow until we have enough strength to release our Father, Zoth Ommog, from his imprisonment beyond the curtain of time.”
A holographic warning“We have at last imprisoned the Dark King, Ythog-Nthlei, here beneath his very base. The process cost more than we could have believed in time and lives. For over 200 years our brotherhood of magicians has struggled to prevent these sons of the old ones from releasing their horrible sires. Now only a few of us remain, but the task is complete. This image is a warning. Do not, under any circumstances, attempt to enter the chamber beyond this door. Any transgression into the area will release the King from his bondage, and will surely spell your doom.”
The Eldritch TetrahedronThis area served as a communications link between the members of the base crew and their leaders. Through psychic stimulation, the device can be activated, as displayed by the increased internal luminance when the party studies the pyramid. The relic was also used as an interdimensional portal, which transported the inhuman creatures’ brethren across space and time to assist them in their unholy cause.