OK so I just checked the files so this should be correct. To get Darklings you need a "Darkling Camp" Improvement. The Darkling Camp will automatically create 3 Groups (units of 4 darklings)... note that they don't get built at the city... they appear on the Darkling Camp itself.
You can only build the Darkling Camp on a "Darkling Village" resource.
But here's the bit that makes them rare... from what I can tell in the files, you can only get a "Darkling Village" resource, but clearing out a Darkling Shaman Lair. The thing is a Darkling Shaman lair is a rare occurrence. Looks to me they don't even spawn on the map by default. They are an upgrade from a Darkling Rider Lair (so a Darkling Rider Lair will turn into a Darkling Shaman Lair after 100 turns).
So they're actually pretty rare when you look at it that way. I think from memory i've only ever had 1 Darkling Camp... and that was in a game where I had a small Darkling lair somewhat near my startpoint.... but it was hard to get to, kinda out of the way so I just left it until late game. I guess by the time I got around to clearing it out it must have upgraded all the way to a Shamam Lair because I was able to build a Camp there.
I'm 95% sure this is all accurate. Happy for someone who has more gametime than me to correct me if any of that seems wrong.
BTW if you really are super keen to build Darkling Lairs, you could mod the CoreGoodieHuts files and have the DarklingWarrior goodie hut drop a Darkling Village (like the Shaman level lair does).