Temple of Id, "Egg-o" and Thanatos.

[Havard]

Perhaps the brute-men/neanderthals were but the first generation of Id Cultists. Maybe the Temple was one of Thanatos' first efforts to revive the Burrowers, and in so doing, they discovered (or drew the attention of) the EGG, only to be defeated before bringing about its rising/awakening?

This fits extremely well with what I was thinking too!

With the "Sleeping Ones" concept, I introduced the idea that the Egg is just one of several such beings so not everything has to be about the Egg, but I definitely see a connection between the Brute-Man Burrower worshippers and the Id Cult.

Thanatos might not care too much if it is a Burrower or an Egg of Coot that is awakened. However, it would have been preferable to him if the Egg was awakened under the control of his own servants. As it ended up, the Egg is now working towards its own goals. In the long run, Thanatos will always win, but at least on the short term, that is something the heroes of Blackmoor can use to their advantage.

-Havard