they are at Sky! not on ground so Go to Portal in Shadow Scion, near them, there Tiny NPC and stand near him , this will send you to Sky.Akvan Horror are around Unwoken Road in Sky not Ground!
Here is a video I made for the carnage quest.
Video: Rift - Carnage - Akvan Horrors
Just avoid their cleave attack and hit them with everything you've got.
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