Contract Spirits and Spirit Sorcerers

Itsugi, who had a weak constitution, falls down the stairs one day. When he next wakes up, he's in a different world of swords and magic!? In that world, he has become slightly younger, and he possesses an impossible status where his weak constitution has disappeared. After contracting with and dominating the divine spirit that picked him up, he lives on in this otherworldly setting in that manner. ※The title occasionally changes.