If you use Bane and it doesn't hit anyone, it won't count. If you use Fester and there weren't DoTs active (or you used it before the DoTs truly registered on the boss), it won't count. If the boss was otherwise invulnerable, ED/Fester/Bane/Painflare won't count.

Basically whatever you use has to do its intended effect for it to give Aethertrail.