Basically, the title, the "feed the void" aspect on warlocks, feels pretty lackluster, especially when comparing it to a fragment that has so much more usability. When using "feed the void", devour should grant the benefits of the "dark matter" perk from the void warlock middle tree, where getting a void ability kill would now grant melee and class energy in addition to health and grenades. That would definitely make it stand out from the fragment.
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Definitely agree with how feed the void feels like a fragment. I also think, maybe a good idea is for the fragment itself to get a nerf (it is a main stay in almost all non-warlock loadouts). Make the extension of devour with kills limited to the aspect which means limited befit from health and grenade energy is still available through the fragment, but consistent use of the buff requires the aspect. Dark matter would be pretty cool though. Kind of like nezarec's sin at base which fits void nicely.