Musai no Shoukanshi

"Summoners" call forth otherworldly powers from a different realm using rings. A summoner's abilities are determined by lineage; the protagonist, Minato Minase, hails from an ordinary family and is unable to summon any phantom beasts. In contrast, his childhood friend, Yukimi Tono, is an elite prodigy who commands a powerful phantom beast. As the gap in their talents widens, Minato continues a reckless training regimen using a sword, determined to one day stand beside his childhood friend once more. Then, one day, the two find themselves in a desperate, life-or-death situation during a small-scale subjugation mission. Just as they brace for the end, a mysterious woman named Azami appears! "I shall lend you my power. The price is..." In exchange for overwhelming strength, the "cost of the contract" Azami demands is not the sacrifice of his life or heart, but something truly unprecedented and utterly unexpected...!?

(Source: Sunday Webry, translated)