It's All Mike's Dying Dream.Mike and Jody both had a car accident, and Jody is lying there dying or he's at the hospital. The Tall Man is just a representation of the death that he has been denying: an undertaker. Some of his last lines in Phantasm have him say "I'm dying, Reggie."
Mike is a Reality Warper.And so is the Tall Man, which is the reason that the Tall Man is so interested in Mike. The events of the first movie did indeed occur, with Reggie being stabbed and the funeral home being sucked into another dimension, only Mike brought it back to normal with his powers. He wanted Jody back and so he brought him back as a sphere. Unfortunately the Tall Man is more skilled at it, and can keep appearing every time Mike changes things. The ending of Oblivion is not a flashback, it's Mike turning back the clock to a more pleasant time, and denying the death that was waiting for him.
The Tall Man and Mike are one and the same.At the end of the fourth movie, after The Tall Man has extracted the golden ball from Mike, he looks really pensive for a bit, before entering the portal. This is because he is reflecting on the event that amounts to his birthday. The golden ball has always been The Tall Man, he "made it" out of Michael, and then took it to the past and put it inside Jebediah Morningside to turn him into himself. That is also why Mike can't hurt Jebediah, or be part of the destruction of The Tall Man, that would prevent him from existing, at least in the form in which he could be capable of hurting Jebediah or participating in the destruction of the Tall Man. The Tall Man of course exploits this for his benefit; as long as Mike, or anyone who has been influenced by Mike, tries to destroy him, he can keep coming back, at least until he gets the golden ball out of Mike.
Tall Man wanted Mike as a successor.In the beginning the intial plan was to kill Mike because he knew too much. Not that it mattered, since people would have found him out eventually. However after years of out gunning and out running him he decided that Mike might succeed in his goal where Tall Man has somewhat failed. Hence why he placed the Golden Sphere inside of him. When he took it out of his head, he was killing Mike to test if Mike would come back the way Tall Man did.
Phantasm is the earliest Star Wars prequel.The Tall Man is the first and the last Lord of the Sith, and he has taken Mike as his apprentice. The Tall Man is probably the very last Lord of the Sith, who has used time travel to go the time when humans lived only on Earth -which would mean he traveled to his objective future, but subjective past-, to try to give the Sith and extra early start. His base of operations is either a pre-colonized or post-apocalyptic Tatooine, which would make his minions either the predecessors of the Jawas or creations inspired by them, or most likely both, since it's all a big Timey-Wimey Ball.
The Tall Man is one of the Mi-Go.Hmmm. He extracts brains and places them in silver spheres. The Mi-Go extract brains and place them in silver canisters.
The Tall Man is The ThingThe portal Jebediah Morningside built sent him to the Thing's homeworld, where he was assimilated. What came back was an imitation. When Mike damages it's hand, yellow blood came out, and it even made the same otherotherworldly scream.The moving severed finger proves that each part of him is and individual. Like the Thing, the Tall Man can shapeshift. Throughout the films it is implied that what is seen is an alien Eldritch Abomination, like the creature. He is taking over the world the way The Thing found in the Antartctic couldn't.