If you allow Master Mirror to take Olgierd's soul, and you choose the first option: "To be as swift as the wind". Equipping it transforms Roach into a demonic looking black horse with glowing eyes. According to the description, it also applies a short-range effect, possibly similar to the stagger caused by the Axii ability Delusion.
The saddle appears to be fashioned of strange leather, the hide of some unknown beast. As evidence of its unusual nature, suffice it to say that it is always rather warm to the touch. The saddle addles the mind of a randomly-selected opponent within a limited range.