Reformatory/1.03.24

Dark Elf
Up until tier 3 the Dark Elves wouldn't have had any way to fill their mines, unless they started the game with any Sorcerors. The Reformatory offers one of two ways obtained at tier 3 for the Dark Elves to produce mine fillers, and arguably the better of the two. Once the Slavehorde skill has been researched, the Reformatory can "summon" up to 10 Thralls immediately, which can then be sent to over-populate a mine. Since the ability has no time delay to be used again, you can then just re-cast it and have enough Thralls to fill all of your mines then and there! At this point the gameplay changes, matching that of the typical races of the game, but yet still maintain the extra map control from Phoenixes and extra crystal income through Ancient Wisps. If you didn't start the game with any Sorcerors, or simply lost them over time before you could get all your mines filled, then the Reformatory should be the first building to build when you hit tier 3, and even worth speeding through the tiers to get.

The building itself is similar to the Tree of Pain in the sense that it's cheap, yet durable and not even that big - about average in size if not smaller. Still, only one of these is really ever needed, but the Slavehorde ability still give it use once all the researches have been had.