I have recently bought both Horror Adventures and the Villain Codex on a whim. However both seem to be perfect for Kingmaker.
From the Horror Adventures, I would use a mixture of Dark Fantasy and Gothic Horror to spice up Kingmaker. If you include Carnival of Tears, Realm of the Fellnight Queen, and Courts of the Shadow Fey, and play them for Gothic Horror, it will only add to the fun. Whether you want to make the Stolen Lands cursed by Nyrissa may be a step too far but would be interesting. Perhaps the PC's have to find some cursed items in Book 1 culminating in the Stag Lords Fort to free their part of the Stolen Lands from the Curse. Each part of the Stolen Lands has it's own part of the curse, although Varnhold was already cursed by the presence of Vordacai. Book 3 is perfect for horror as is Book 4 if you make the Tiger Lords particularly vicious and gruesome.
As an example of what you could do, the opening scene at Olegs could actually start as the players arrive at the gates. The bandits are already there and are tasking Svetlana as a slave, because business has been slow and Oleg does not have the money for a few days. The Stag Lord could be a slaver as well as a bandit. I would replace Nugrah the Discrepit with the Experimenter from the Villain Codex to make the Fort even more horrific.
I would also use the Syndicate idea from the Villain codex to show how desperate things have got in Rostland. It would make the start of Book 2 much more interesting! The Regal Court could also be used for Nolski Surtova and his court, with his ambitious sister as the Queen. This would give a whole new political element to the game if you want to include some Brevoy politics.
