Magic Knight Rayearth: Three girls are transported to a mystical realm and given magical powers. They can summon the battle armor Rayearth as well. They set out to save a priestess supposedly being held captive. They fight her captor, only to learn that the priestess wasn't being held against her will but was trying to free herself from her enslavement to the rest of the world. So the three girls are forced to kill the priestess, which leads to a civil war in the mystical kingdom. And I forgot the rest of the second arc.
Slayers had Rezo the Red Priest. He wanted more power so he could cure the ails of the world, so he delved in various dark arts (in spite of the fact he was a priest), turned a young boy into a golem-human chimera, and laid waste to an entire city. He eventually became consumed by the powers of the dark lord Shabranigdo and asked Lina inverse to free his soul from Shabranigdo's clutches in exchange for saving a kingdom from a deadly plague, resulting in the death of Rezo.
Urotsukidouji was about three universes - the human world, the demon world, and the man-beast world - awaiting the coming of the Overfiend, a powerful being believed to unite the three worlds. The humans that knew of the legend sought the Overfiend for power, the beast-men sought it for peace, and the demons didn't want the Overfiend to appear because they liked things as they were and foresaw the Overfiend's arrival as heralding the destruction of the worlds. The Overfiend was to arrive in the Human world, so the man-beasts sent agents there to track him down. They tracked it to a boy named Nagumo who was hit by a truck and died, but was resurrected good as new by the powers of the Overfiend. So the man-beasts protected Nagumo from the Demons until his powers awakened. Unfortunately, Nagumo wasn't actually the Overfiend and turned into a giant monster that laid waste to Osaka, slaying countless humans, demons and man-beasts. The Overfiend was in fact the offspring of Nagumo and his girlfriend Akemi. (So the boy had awesome powers and was supposed to be a hero but was in fact an evil being.)