Dunno whether it is a bug or I am missing something. But after killing all guardians including the Death Demon and seizing all lairs and the dark spire I still can't settle or build an outpost in the Imperium wildland.
Funny, I had exactly the same issue in a game yesterday. Cleared out all the wildland monsters (Obsidian Golems, Death Demons), took the dark spire -- but still could not settle or build outposts in the Imperium ruins. Got the little quest box popping up on the left margin; but could never complete it. I had a couple of my own cities close to the Wildland border, but I don't think they were close enough to interfere.
Is the Wildland supposed to respawn arable tiles when you clear it? Or just one tile? Not sure how this works. Was never able to complete the quest, or farm the shards within the Imperium borders. A bit frustrating.
Were you able to settle once the neighbouring cities were removed?
This issue was listed under Fixed in the 1.3 Changelog... but I dunno...