I think the best way to fix this, and the route I am sure they will go, is to make it an invocation that recharges on a long rest, or one of those 1/day shenanigans, probably with pact of the Chain as a prerequisite. As for "does it fit the fiend" Personally I hope they leave it up to the player to decide if it makes sense for their character. I could see the Prince of Frost or the Queen of Air and Darkness promising they will reveal the secrets for conjuring demons, not because they are demonic themselves, just that in their long lives and quest for eldritch secrets they discovered the true name of a Hezrou or two.