This one is fairly straightforward - just kill skeletons and shut down 5 of their spawn points. Hopefully you'll start getting this type of enemy spawning to attack your settlement so the 100 skeletons shouldn't be too difficult - it's finding the Abandoned Graveyards that can be challenging. I had long completed the tech tree before I managed it on my game. They are only found in the Dire Tunnels biome, so as you're exploring if you see that you're going through "Tormented Soil" make sure you explore that area thoroughly to uncover any graveyards.