Ancestral Hall/1.03.24

Dark Dwarf
The Dark Dwarves don't usually make much use of their crystal, but the Ancestral Hall changes that. Not only can it produce Wraiths and Shadows normally, but it can also summon them instantly - although at a much more hefty cost of crystal. Because of this, you wont really need any more than just a couple of these buildings if you do plan on using Shadows. The building can also research skills that can buff Wraiths and Shadows more than the Undead can, despite Wraiths and Shadows being an Undead unit. Like Wyverns, Shadows (especially after upgrades) can help Dark Dwarves out with the helpful speed that the core of their army lacks. That doesn't mean that Shadows are a compulsory unit needed to make the race function, and could even be skipped altogether.

Whether you choose to use Shadows or not, you'll find yourself building this building anyway for a more important reason - Earthpower. This skill, once researched, allows Engineers to cast the spell with the same name - instantly and completely repairing all buildings nearby for no cost other than regenerative mana! Quite an impressive feat for just a builder, eh?