The Mystical Sack is a magical bag that can catch and hold any creature its owner commanded.
In an old Russian folktale, a strange beggar rewarded a soldier's kindness by giving away his bag. The beggar instructed the soldier on its powers then vanished. At first, the soldier used the bag to catch animals to kill and eat on his way home. With it, he helped others and became a rich and adored man. The soldier then used the sack the battle an Underworld Horde. He captured Death and became invincible. He drove the horde back to the underworld. However, when he became old and sick, Death refused to kill him and cursed him to wander the earth. The Undying Soldier vowed never to use the sack again and retired to a tower in present day Siberia.
In the 1970s, a Senn University student named Eugene Visitor used his backpack in a similar fashion as the Mystical Sack to trap Death. Eugene vanished and reappeared 20 years into the future. It became apparent only Eugene could open it. He declared he would only think about opening it if Egon Spengler returned with concrete proof about the catastrophic damage he was causing by keeping Death in the bag.