Which wildland?

In Curgens tomb you have to kill all enemies and also gather all treasure chests (i.e. shrill lairs) ...

in the huge battlefield IIRC you have to kill the golem armies and move through the dale from its entrance to its exit (there are 2 special tiles one on the entrance and one on the exit, which you have to visit)

in the wildland with the huge number of caves (that were used as a torture chamber) you have to visit all caves.

Not sure about the other wildlands, but if you kill all moving enemies AND clear all special tiles, you should usually convert the wildland into a normal region.

Yes, it's the swamp.  Thank you!  I'll look closely.

I can confirm that killing the boss does not convert the swamp, as I thought this would help with the plethora of Haunter armies. I went after the him asap. The swamp remained until I had explored the special tiles.

the various different wildlands have different conditions for clearing them. 

Some require you to kill all enemy armies (possibly including ones that might have wandered out of the area)

Some require you to only kill a specific boss monster

Some require you to investigate all caves/treasure areas

And at least one (Imperium) requires you to found a city inside it, on a specific spot (it's the only buildable tile inside the walls)

In any case, you'll know when you've completed the wildland, as there will be a massive explosion of light in the centre, and the dark green cultural borders around the area will vanish. Until that happens, it's not complete.

And if a town has been built (either by you or the AI) within 6 tiles of this specific spot, you can't clear the wildlands because you can't build on that tile. Only workaround is to raze the nearby town.