When Ancestral Vengeance enters the battlefield, put a +1/+1 counter on target creature you control.
Enchanted creature gets -1/-1.
11/24/2014 If the creature targeted by Ancestral Vengeance’s enchant creature ability is an illegal target as Ancestral Vengeance tries to resolve, Ancestral Vengeance won’t resolve. It won’t enter the battlefield, and its enters-the-battlefield ability won’t trigger.
11/24/2014 If Ancestral Vengeance causes the enchanted creature to have toughness 0, that creature and Ancestral Vengeance will be put into their owners’ graveyards. The enters-the-battlefield ability will still apply.