When he reveals himself in his incomplete form to Yami Yugi. Anubis was shown as a tanned and muscular but pulsating man with dirty blonde hair, red eyes, and a jewel embedded in his forehead. His clothing consisted of a black cape attached to spiked padding which were held by strings, along with gray shoulder pads, black pants, and the Pyramid of Light hanging around his neck.
In Egyptian Mythology, Anubis is the Egyptian god of embalming, possessing a jackal head and was said to have created the first Egyptian mummy. Anubis was originally the god of the Underworld and of the dead (later replaced by Osiris). Both of these are similar to character in his title of "Lord of the Dead" and his complete form's jackal-like head, along with his ability to control mummies. However the Egyptians never viewed Anubis as evil, and he was, in fact, one of their most respected and revered gods due to his role of bringing the dead to the afterlife.
When Yugi had a vision after being driven unconscious by a strange force when looking at the pyramid, he saw Kaiba being grabbed by Anubis who had a black jackal-like head with a white muzzle instead of his human head and his was not pulsating, possibly being Anubis' complete form, if he had successfully drained all of the energy from Yami Yugi and Kaiba.
After losing the duel to Yugi, Anubis used the power of the center jewel of the Pyramid of Light to assume a monster form that started off as slime until forming an armored jackal-like monster that could stand on his hind legs, possessed bladed wings, and an axe-like blade on top of his head, and the Pyramid's jewel in the center of his chest.
5,000 years ago, Anubis tried to destroy the world, but was destroyed by Yami Yugi, the Pharaoh, though a prophecy told he would return from the dead.
When Yugi solved the Millennium Puzzle, not only did Yami's spirit awaken, but also Anubis's spirit inside the Pyramid of Light. Anubis's spirit placed the Pyramid of Light card in Maximillion Pegasus's deck, which, along with the Blue Eyes Shining Dragon, was taken by Kaiba when he beat Pegasus in a duel.
Kaiba dueled Yami Yugi, and played the Pyramid of Light card to destroy his Egyptian Gods. Playing this card empowered Anubis to suck Yugi, Joey, and Tristan's souls into the Millennium Puzzle, where they were attacked by mummies. When Yami or Kaiba lost life points in their duel, their life force was sucked out and given to Anubis, who gave Kaiba duel commands, but Kaiba resisted and tried to destroy the Pyramid of Light so he could beat Yami with his own Egyptian Gods, but was unsuccessful.
Anubis then appeared in physical form, restored to life, and knocked Kaiba out. Anubis continued the duel against Yami, but when Yugi blinded Anubis's eye inside the Millennium Puzzle by throwing the Dagger of Fate at it, the Pyramid of Light was weakened, enabling Yami to destroy the card. Anubis summoned an overpowered monster, but Yami revived the Egyptian Gods, who destroyed the monster, defeating Anubis. The actual Pyramid of Light was destroyed, and seemingly, so was Anubis. Yugi, Joey, and Tristan's souls were released.
However, the orb of the Pyramid of Light, containing Anubis's soul, survived, and Anubis transformed into a monster, telling Yugi it was time to die. With Yami at his side, Yugi summoned Blue Eyes Shining Dragon, given to him by Kaiba, which had the power to destroy any monster. Blue Eyes Shining Dragon used Shining Nova Attack to destroy Anubis once and for all, along with the orb of the Pyramid of Light.
Anubis' powers are connected to the Pyramid of Light. With the item, he is able to have his spirit go wherever needed, transport his mummified body and the pyramid into the Millennium Puzzle, command undead mummies, possess others, and show others the future. When the Pyramid of Light card is in play, Anubis is able to drain life energy from others who have taken damage during the duel in order to restore his body. He is also shown to be able to create cards of his own, creating the Pyramid of Light which he put in Pegasus' Deck, and his Sphinx monsters through a black slime that releases from his hands. Anubis's original form shows him with tan skin, a ragged black cape, and the Pyramid of Light. When in his monster form, Anubis is able to breathe a devastating black flame from his mouth and can easily resist other monsters' attacks. In his presence, Duelists can summon Duel Monster cards that have real powers, including their special effects, as Joey Wheeler was able to summon "Gilford the Lightning" and "Gearfried the Iron Knight" to battle Anubis, Maximillion Pegasus was able to summon "Toon Dark Magician Girl" and "Blue-Eyes Toon Dragon" to stop falling debris, while Yugi and Kaiba were able to summon the "Blue-Eyes Shining Dragon", whose special effect to destroy any monster brought his end once and for all.
Anubis is often mistaken for being the universe's actual adaptation of the Egyptian God Anubis, however this is only because it isn't established in the English dub that he just happens to be named Anubis and has no connection to the god.