Umbrals. These guys, avaliable to all eras of Agartha iirc, are extremely good for their price. High health, ethereal, life stealing undead are one thing. But when you throw in national mages that can cast darkness and marble warriors or army of gold/lead and it's something else.
However, Umbrals can't be central to your strategy as Middle Era Agartha, IMO. Only national mage capable of summoning them is the capital-only Oracle. If they've got the spare time, its a very good use of their summon, but unless you get (onyx?) amazons or the right independent mages, it just costs way too much to get going as a central part of your strategy. Quite a bit more viable in LA or EA.