Spawns a familiar that drops a Heart, coin, Bomb, or Key (or any of their variations) alternatingly every 5 or 6 rooms.
Notes
Familiars that follow behind Isaac have a certain priority in how close their position is to him over other familiars, with Mystery Sack having the lowest priority at 6th / 8th and thus being farther from Isaac than familiars of different priorities.
Can spawn troll bombs.
Synergies
BFFS!: The familiar drops pickups alternatingly every 4 or 5 rooms.
Before patch 1.04, this item did not drop any items or do anything at all. Why it did not work remains a mystery.
This item is the only "bag" item (Sack of Pennies, Rune Bag, Bomb Bag, and Sack of Sacks), that isn't a counterpart to other existing bag items, rather being a combination of some.
Bomb, or 
Key (or any of their variations) alternatingly every 5 or 6 rooms.
Familiars that follow behind Isaac have a certain priority in how close their position is to him over other familiars, with Mystery Sack having the lowest priority at 
6th / 
8th and thus being farther from Isaac than familiars of different priorities.
BFFS!: The familiar drops pickups alternatingly every 4 or 5 rooms.
Sack of Pennies, 
Rune Bag, 
Bomb Bag, and 
Sack of Sacks), that isn't a counterpart to other existing bag items, rather being a combination of some.
Eden Unlocks 
The Lamb Unlocks 
The Binding of Isaac: Rebirth
