U.n.k.n.o.w.n.
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Regarding Grol, maybe we could base him on the barbarian hero (skeleton and animation)? Replace his head with Agrael's helmet, recolored to black; recolor the axe to black or change it to another one to make it look like the axe from the artwork. As for the bull, I don't know what to do; here are some possible options: try to have the hero ride, for example, a hell steed/nether horse or a hell hound/fire hound as an effect if it's not possible to fully attach it to the bull's skeleton. Another option is to make the bull look like a hellhound from Warcraft III.
Added some time later...
As for the female heroes, some of them could be based on (skeleton/animation) the knight hero, Isabel/Frida, and/or the sorceress hero, Illaya/Shadia, with recolored textures; attaching horns to the head or replacing the head with another one; replacing the horse with a nether horse (for the hero based on the knight), etc.
Added some time later...
As for the female heroes, some of them could be based on (skeleton/animation) the knight hero, Isabel/Frida, and/or the sorceress hero, Illaya/Shadia, with recolored textures; attaching horns to the head or replacing the head with another one; replacing the horse with a nether horse (for the hero based on the knight), etc.