You may cast Eternal Scourge from exile.
When Eternal Scourge becomes the target of a spell or ability an opponent controls, exile Eternal Scourge.
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.
You must follow the normal timing permissions and restrictions and pay its cost to cast Eternal Scourge from exile.
You’ll exile Eternal Scourge even if the spell or ability that targets it is countered.