I'm about to make your day friend:

Meet Eslek Tarosh[endlesslegend.gamepedia.com]. The Haunt hero. I don't know the EXACT lore, but he's closely associated with Endless Day, the fictional holliday that permeates the Endless universe. You can unlock him by setting your computer clock to 1/21/16-1/26/16 and doing the quest you get from the ruins. After you do that, he has a small chance to show up in the market, like other heroes.

He has _two_ unique skill trees, being the only Haunt hero, and the only flying hero. Both of those trees seem to focus on increasing initiative and damage, while the Haunt Tree is also kind of sciency, and gives him access to disease (which can destroy your own army pretty quickly sadly), and supercharged chain lightning.

He can be very powerful, but I find that unless you send him in with some healers, or you have empire regen from some source, he tends to take more damage than he deals (no access to shields/boots for additional armor). Whenever I'm lucky enough to grab him early, I find he tends to serve as a kamakaze hero. He's goes first, hits something for an ungodly amount of damage, chainlightnings the rest of the enemies, and then dies, reviving with last stand after the battle.

It would be difficult to make minor faction trees simply because of how quickly that could imbalance things. For example if you think about the Delvers, you would want their hero to be a strong infantry hero who could also help generate dust right? Well with than in mind, how do you differentiate them from the broken lords heroes then? I think they felt comfortable giving the Haunt hero two unique (and very powerful) trees because he's relatively hard to get.

The one exception to this I think would be a Dorgeshi Hero. The Dorgeshi are a pretty unique faction in that they have a very interesting/useful unique charge, serve a distinct purpose (initiative locking/reinforcement blocking), and their governor talents could focus on luxury benefits/extraction.