Slumbering Dragon can't attack or block unless it has five or more +1/+1 counters on it.
Whenever a creature attacks you or a planeswalker you control, put a +1/+1 counter on Slumbering Dragon.
7/1/2012 Slumbering Dragon’s second ability counts all +1/+1 counters on it, not just the ones added by its third ability.
7/1/2012 The +1/+1 counter is put on Slumbering Dragon before blockers are declared. This may allow Slumbering Dragon to “wake up” and block the creature that attacked and caused its ability to trigger.
7/1/2012 Slumbering Dragon’s third ability will trigger once for each creature that attacks you or a planeswalker you control.