OK, so...
"Hypnotic Confusion" (Su): A ghostlight can brighten and dim its lights to confuse its prey (as the confusion spell) for 2d6 rounds. Alternatively, the ghostlight can alter the pattern to fascinate victims (as the hypnotic pattern spell) for 2d6 rounds. Any creature within a 30-foot-radius who can see this display must succeed on a DC X Will save or suffer the chosen effect. This is a mind-affecting, illusion effect. The save DC is Charisma-based.
Here's the yakfolk ability...
Body Meld (Sp): Once per day a yak folk can merge its body with that of a humanoid or giant of Small to Large size. Except where noted here, this power functions like a magic jar spell heightened to 9th level (caster level 20th; Will save DC 21 negates). To use this ability, the yak folk must touch the intended target for 20 minutes without interruption (yak folk usually restrain their victims while using this ability). At the end of this period, the target must make a Will save to remain conscious. On a failed save, the host's mind becomes unconscious, and the yak folk assumes control. There is no magic jar per se--the yak folk physically merges its body with the victim's body without the use of an intermediate vessel. Body meld lasts until dispelled or dismissed.
The process works only on humans, elves, dwarves, half-orcs, and any creature of the giant type. The yak folk shares all the victim's knowledge, memories, skills, feats, and extraordinary abilities, but none of its spell-like or supernatural abilities. The yak folk retains all of its own supernatural and spell-like abilities. The merging does not shed a magical aura (so a detect magic spell does not reveal it), but a true seeing spell reveals the victim's dual nature.
If the host body is slain, the yak folk dies with it. Separating the two bodies takes half as long as melding (10 minutes). The host regains consciousness 1d6 minutes after the separation is complete.
Only characters who know the victim personally have any chance to realize that something is wrong, by making a Spot check opposed by the yak folk's Disguise check (which in this case represents how well the yak folk impersonates its host).