2016-07-13
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.
2016-07-13
You must follow the normal timing permissions and restrictions and pay its cost to cast Eternal Scourge from exile.
2016-07-13
You'll exile Eternal Scourge even if the spell or ability that targets it is countered.