Phantasm III: Lord of the Dead

Immediately after his apparent demise at the end of Phantasm II, a new Tall Man emerges from a dimensional portal. At the same time, the hearse that carries Liz and Mike explodes. Reggie finds Liz dead but saves Mike from the Tall Man by threatening to kill them all with a grenade. The Tall Man retreats with Liz's body and threatens to return for Mike when he's well again. After Mike spends two years comatose in the hospital, he has a near death experience in which his dead brother Jody and the Tall Man appear. As he wakes from his coma, he is attacked by a demonic nurse, but Reggie appears and helps him to fight her off. Back at home, the Tall Man arrives via dimensional fork, fights off Reggie, transforms Jody into a charred sphere, and draws Mike through the gate with him.
The next morning, Reggie (with the Jody-sphere) travels to a deserted town and is captured by three looters, who lock him in the trunk of the Hemi-'Cuda. Reggie is rescued by a young boy named Tim, who kills the looters when they break into his house. After they have buried the looters in the yard, Tim tells Reggie how the Tall Man took his parents and destroyed the town. In the morning, Reggie and Tim find the three graves empty and their hearse gone. Reggie tries to leave Tim with an orphanage, but the boy hides in Reggie's car. Reggie enters a mausoleum and is confronted by a sphere, but he is subdued by two young black women, Tanesha and Rocky, before he can destroy it. Reggie tries to warn them, but Tanesha is killed by the sphere. Tim appears and destroys it with his pistol. The three join forces, come upon a convoy of hearses driven by Gravers, and decide to follow them.
At night, Jody appears to Reggie in a dream and takes him to the Tall Man's lair, where they rescue Mike. As Reggie wakes, Jody opens a portal and Mike emerges. The Tall Man tries to follow, but Reggie closes the portal, leaving the Tall Man's hands behind. After fighting off the Tall Man's minions, including the undead looters, they enter a large mortuary. Inside, they find a cryonics facility, and Mike remembers that the Tall Man dislikes cold. While Reggie, Rocky, and Tim are separated and attacked by the looters, Mike consults with the Jody-sphere, who explains that the Tall Man is amassing an army to conquer dimensions: brains are harvested to turn into the killer spheres, and the bodies are shrunken and turned into drones. The Tall Man senses their presence, captures Mike, and straps him onto a table. Two of the looters wheel in Tim. Mike tries to give a message to Tim, warning him that "there are thousands of them", but Mike is paralyzed by the Tall Man.
Meanwhile, Rocky defeats her attacker and helps Reggie. Cut free by the Jody-sphere, Tim runs into the remaining looters, who are killed by the Jody-sphere and Reggie's 4-barrel shotgun. The trio crash into the embalming room, where the Tall Man is operating on Mike. Rocky impales the Tall Man with a spear dipped in liquid nitrogen, and they lock him in the refrigerator room. However, a golden sphere breaks out of his head and attacks them; Reggie catches it in a plunger and, with some help, manages to dump it into the nitrogen tank. Mike finds a golden sphere in his own head, and his eyes turn silver. Complaining of the cold, he leaves with Jody and warns Reggie to stay away. Reggie suggests exploring the mortuary, but Rocky declines and leaves too. Tim reports that Mike tried to warn him, but they find out too late that there are dozens of spheres left, and Reggie is pinned to the wall by them. A new Tall Man reappears and watches as Tim is pulled through a window by a creature, his fate unknown.

Question: What is being harvested?
Answer: brains