Finding those fics where the summoning acts as a catalyst for deep personal change—it's tricky. The obvious ones, like 'The Denarian Renegade' series or 'Wastelands of Time', get mentioned a lot. They're classics for a reason; Harry's merger with a Denarian coin or his bargain with powers beyond comprehension force massive shifts in his morality and identity. But the transformation often feels external, powered by the entity. I keep looking for stories where the demonic pact is the match, but the fuel for the change is something already rotting inside the character.
There's a quieter one I stumbled on ages ago, 'A Black Comedy' isn't strictly about summoning, but the infernal alliance there alters Harry's and Sirius's entire worldview in a darkly comic way—they become something else, thieves in a new world, their morals sanded down by convenience and power. For a more psychological unraveling, 'With Strength of Steel Wings' is an old, unfinished gem. Harry's obsessive research into arithmancy and ritual to gain power, which includes summoning, transforms him from the inside out long before any external entity fully manifests. He becomes isolated, paranoid, a different person to his friends, which is a transformation more chilling than any physical possession.
The real character studies, for me, are in the fics where the 'demon' is less a monster and more a mirror. A one-shot I can't recall the name of had Hermione summon something to protect Hogwarts after the war, but the cost was a piece of her empathy. She won, but became cold, strategic, and distant—a transformation she welcomed because it made her 'effective'. That loss of self, willingly traded for purpose, is the most demonic transformation of all.