Mystical Arachnid

"That wasn't just a key. It was a Guardian, designed to prevent Apocalypse's release"

—Charles Xavier

The Mystical Arachnid was created to guard Apocalypse and make sure that he was never awakened. It was a giant, mystically-empowered, green spider, and the guardian of the Second Door that sealed Apocalypse's tomb.

The Mystical Arachnid was sealed within a spider-like artifact made up of two parts, which were then separated. Reuniting these two parts then summoned the Arachnid.

In a sense, the Mystical Arachnid was the last line of defence to prevent the opening of Apocalypse's tomb. Due to the manipulations of Mesmero, the Arachnid was summoned. Deceived by Mesmero and against the advice of Charles Xavier, Magneto attacked and destroyed the Arachnid, thereby opening the Second Door.