The defenses of the Abzan extend well beyond the walls of their fortresses.

Ancestral Vengeance

Enchant creature
When this Aura enters, put a +1/+1 counter on target creature you control.
Enchanted creature gets -1/-1.

The List

Fate Reforged

Enchant creature
When Ancestral Vengeance enters the battlefield, put a +1/+1 counter on target creature you control.
Enchanted creature gets -1/-1.

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.

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.