Raiding villages:
1) Go to map.
View attachment 4631734
2) Select a village, click on crossed swords, and you'll see raiding interface, like this:
View attachment 4631735
3) Drag and drop one of the parties from the left to the center window. The result should look like this:
View attachment 4631736
4) Press "execute".
The interface will close, and the icon will darken, turning into a progress clock:
View attachment 4631737
5) Wait for it to finish (you can do other things at your base in the meantime):
View attachment 4631738
6) See the spoils of your victory:
View attachment 4631742
Right-click each of them to take them home. Left-click to see their stats.
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Raiding monsters:
1) Go to map.
2) Select a monster lair, and drag and drop any slaves to their window:
View attachment 4631743
3) Swords icon will light up. Click on it, and do steps 3+ for raiding a village.