The spell made the target appear as a vile yak-man as far as all of the physical senses were concerned. Only a true seeing spell could piece this illusion. This spell could only be dispelled through a wish. This illusion was so strong that even creatures, such as undead, that were normally immune to illusions, were affected.
While under the guise of a yak-man, the target would never be harmed or hindered by a dao. However, most civilized folk of Zakhara despised the yak-men, so the target could not encounter other humanoids without being attacked on sight.