The Star of Xil'yeh
Grel'borg the Miser has the Star. He is a greedy ogre who spends his days in the Alterac Mountains, in the Ruins of Alterac, searching for baubles. Most of his collection is useless, but one item, the Star of Xil'yeh, has valuable properties.
The Hand of Dagun
Ancient texts claim that claws of Dagun excrete a poison that does not kill his prey, but mutates it into something else -- a member of the old races. Its value to us is unquestioned.
Dagun lives in the deep sea, but is regularly enticed to the surface by a tribe of Mirefin Murlocs in Dustwallow Marsh. Their oracles summon him with an enchanted sea kelp. If you kill enough oracles you'll find the kelp. Then place it on the Murloc altar and Dagun will come.
The Legacy Heart
Mogh the Undying is a troll witchdoctor in Stranglethorn. And he posseses the Legacy Heart, said to stave off death to those who can unlock its secrets. You will find Mogh in the Ruins of Zul'Mamwe. Defeat him, if you can, and bring me the Legacy Heart.
Bring me these three items, the Star, and Hand and the Heart, and Yagyin's Digest will be yours.