@Fapxan : He formed a contract with her, twice now.
First time in his past life, where he did it as part of human experiment.
Second time in this life to prevent her from creating more victims.
Though he promise her he'll change the terms in the future to be less restrictive which may require a 3rd contract.
So he, and the other people in the same experiment, used the 'dish' tool to lure out these supernatural beings with the scent of their souls.
The beings give power to them in return for prices, usually their souls.
In his case, Farenna eat souls, mainly 'criminals', though the criteria is based on her judgement (hence that one time she nearly killed those idiots who attacked him, because by the law of yandere waifu, anyone who hurt her man must die)
And this time, because his soul alone wasn't enough, she went to sleep inside him and demand he feed her a dragon's soul.
So she woke up when she found him dying and there's a dragon around, so this yandere waifu ain't having her man dying on her like this, so it's time to anime it up.
His hesitation to use her power is likely because he kinda goes edgelord (note his eyes at the last page) when it happens since she was lured to him by his DEEP hatred and despair, not to mention the whole she eat souls of whatever she want thing.