Ravnican Splicer

This creature has haste, double strike, and trample if it has a +1/+1 counter on it.
Unleash (You may have this creature enter the battlefield with a +1/+1 counter on it. It can't block as long as it has a +1/+1 counter on it.)
Whenever this creature attacks, sacrifice another creature. If you can't, Ravnican Splicer deals 5 damage to you.