Let's say the shadows are sort of dark elves in dark cloaks (but not like Nazgûls or ghosts, not solid) with an innate Shadow Gift that allows them to:
a) fight better.
b) become immaterial with sufficient concentration.
Well, let's put it this way:
- Level 1: warrior of the Shadow Clans. 10% chance to become invisible during an enemy attack. Movement: runs half the field.
- Level 2: Shadow. 20% chance to become invisible during an enemy melee strike and 70% during an enemy ranged shot. Movement: teleportation. Effect is dark, without magical bursts.
It's just that minotaurs are very cute; I wouldn't want to replace them...