an explosive barrel  

In Pellucid Grotto: Part of the event "The Unburrowing" (see this quest entry for details).

In Shard's Landing: Part of the event "Calling Phantasm" (see this quest entry for the group version and this quest entry for the raid version).

In the Bixie Warfront: Part of the heroic adventure "Endless War" (see this quest entry for details).