Sole Defender

When Sole Defender enters the battlefield, if
there are no other creatures on the battlefield,
put 5 +1/+1 counters on it.
When Sole Defender attacks, if it is the only
attacking creature, Sole Defender gains Lifelink
until end of turn. Untap it.
If there is another creature named Sole Defender
on the battlefield, sacrifice Sole Defender.