The Thrall is a unique unit of the Nidhogg clan. They are either slave or serf which acts as civilian units exclusive to the shady Clan of the Dragon, and thus do not count towards your population and do not require housing. Their maximum number is based on your current population number: 33% of your current population. They also consume only 33% of what a Villager usually consumes, and they gather 50% less resources than a civilian.
You can buy them from your Longship Dock. The price of each thrall will increase after each purchase (+40 ) , and this price will slowly decrease over time (-2 /month).
After getting the Capture Lore, you’ll be able to enslave enemy civilians. You’ll need to get them to 33% HP to have a chance (10% chance per hit) to enslave them. This also works for neutral factions. Please note that each enslaved Kobold will take 1 slot in your Thrall population , each Myrkálf will take 3 slots, and each Jötunn will take 5 slots.