Cage/1.03.24

Undead
Like the Barrow, the Cage is a small building that allows Skeletons to morph into a stronger unit (in this case, the Slayer Knight), as well as offering skills to power up those newly morphed units. However, the Cage has another function - the ability to "summon" Thralls. Because of this, and the fact that the skills to research are incredibly expensive, you wont immediately forget about the Cage after building it.