Dominions 3: The Awakening/Mictlan/Mictlipoctli

Commentary
Possibly the only hero available to Mictlan in the Early Era, Mictlipoctli offers an excellent way to "bootstrap" yourself to a sizeable undead force:

Research to Blood-5, and have him cast Contact Civateteo at the relatively low cost of 25 blood slaves. The Civateteo can then start to reanimate (as can Mictlipoctl himself). At five to 10 longdeads a turn, this adds up quickly.

A few turns this and you can get quite a factory of Civateteos churning out decent-sized armies of undead legions quite quickly.