"Tulku" (sprul.ku) is the Tibetan term for nirmanakaya
, a presentation in form as a body.
In the eleventh century, after the death of the Dusum Khyenpa, called the Karmapa (Man of Action), a famous lama of Kagyu lineage of Tibetan Buddhism, the monks of the various monasteries founded by him found the support of their lay patrons rapidly drying up.
They soon found what they claimed to be the direct reincarnation or embodiment or tulku of the Karmapa.
Thus the Karma Kagyu school of Tibetan Buddhism was initiated as was the tradition of passing control over the resources of various associated monasteries and lands that became the trait of all of the other Tibetan Buddhist schools.
Essentially, a tulku was the lord of a particular fiefdom.