What if instead of summoning a demonic tyrant (I thought the warlock was supposed to be the boss here) the warlock instead sacrificed all imps to empower themselves making their abilities hit a lot harder (or have an additional effect) for a period of time depending on the number of imps sacrificed, but your abilities don’t summon demons and don’t cost soul shards. (call dreadstalkers would be temporarily replaced.)
It would be metamorphosis without the actual transformation and you still get to be the summoner archetype. Could even call it demonic sacrifice or something like that. (Although the empowered state would need to be visually impressive.) Also the pseudo-meta that shadow priests have can be deleted.
Likely a disaster waiting to happen but a template for a better idea maybe. I just like the image of summoning an army of imps to fight for me only for me to kill them all myself for my own gain.