You may cast Eternal Scourge from exile.
When Eternal Scourge becomes the target of a spell or ability an opponent controls, exile Eternal Scourge.
7/13/2016 You must follow the normal timing permissions and restrictions and pay its cost to cast Eternal Scourge from exile.
7/13/2016 Eternal Scourge’s triggered ability only triggers while it’s on the battlefield. For example, Convolute cast targeting Eternal Scourge won’t cause its ability to trigger.
7/13/2016 You’ll exile Eternal Scourge even if the spell or ability that targets it is countered.