Glad it's nicely intense!
We are investigating a possible bug or mod interaction that might be causing certain incidents to fire too early. For example, the pit gate is never supposed to appear before the monolith reaches level 2, but from your story it sounds like it appeared right after activating the monolith to level 1.
Can you confirm that the pit gate appeared right after initially activating the monolith? Or was it the similar but different pit burrows? (Pit gate is the one where you have to go underground to defeat it)
And can you let us know if you have any mods active and what they are?
(Might beg for more info/savegame after if that's okay. We're trying to reproduce this.)