

Dream can't touch the black apples, and Nightmare can't touch the golden ones, otherwise they would invert. The apple incident occurred when Dream and Nightmare were six years old, physically, which meant Nightmare went through at least five years of bullying. They were then given skeleton bodies (based on Swap!Sans), which allowed that energy to flow without trouble - this is why they are not considered Sanses per se, as they only inhabit the body of one.

Nim was going to give them human bodies to protect their frail forms, but the energy that radiated from them made them explode as they had very few escape points. Dream and Nightmare were originally little fireballs, yellow and purple respectively. It's possible this human was one of Quetzalcoatl's creations, since at the time of the attack there was no life near the Tree of Feelings, and that it might have been deliberate. Dream and Nightmare were created after their subsequent mother was killed by a human. When Nim sensed there was something or someone within one of the two other worlds that had something against her (it wasn't specified who, but it has been hinted that it was Quetzalcoatl), she decided to move her world away from them, into one of the surrounding multiverses in this case Undertale's. The worlds where the trees reside are actually in the middle of every multiverse, separated from everything and right next to one another. Because of this, Dreamtale occurs in a world completely unrelated to any place from the original Undertale, since it's the occult world where the Tree of Feelings resides. Each tree is in a hidden world nobody must know where they are in order to keep them safe, though some people have found them, both good and bad intentioned. Within the concept of the AU, there are three vital trees that keep the balance of the universe: Life, Magic and Feelings, all protected by extremely powerful guardians ( Quetzalcoatl, Lanny and Nim respectively).
