Enchantment — Aura, BB (2)
Common (C1)
Enchant creature
Whenever enchanted creature becomes tapped, put a -0/-2 counter on it.
Illus. Edward Beard, Jr.

10/1/2009: Spirit Shackle's ability triggers only when the enchanted creature changes from untapped to tapped. If Spirit Shackle enters the battlefield attached to an already tapped creature, it won't do anything (until that creature untaps and becomes tapped again, that is).
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander