Sword of the Waltz

Equipped Creature gets +1/+1.
If a +1/+1 counter is on Sword of the Waltz, it
becomes a 1/1 creature with Flying
At the end of the turn, if the equipped creature
or Sword of the Waltz attacked or blocked, put
a +1/+1 counter on it. Then, if there are 5 or
more +1/+1 counters on it, remove all counters
and attach it to a creature you control.
Equip {2}
EE - I'll give a favorite to the first who solves the riddle of why 4 counters is the limit.