Haramase Legend Shoukan! Zettai Reido no Bakunyuu Himekishi o Niku Onaho ni Suru Nariagari Isekai Seikatsu

The protagonist is a servant serving in a certain temple.

One day, using a rare treasure called "Ticket", a ritual is held to summon the great heroes. But during the ritual, an evil spirit Giselle, attacks the place. The protagonist, who is peeking in on the ritual, also gets caught up in the commotion.

Rather than getting killed, the protagonist uses the Ticket, and is able to summon the best and rarest being from the spiritual world whom he has been longing for, the princess knight Rufina. After defeating Giselle and settling the situation, Rufina challenges the protagonist to a match to see if he is qualify to be her master.

Nothing could beat the perfect well-rounded Rufina, so he gets desperate and pushes her down, deciding to challenge her in sex. Although Rufina sees through his wicked desires and despises him, she is getting banged so hard that it brings out her masochistic side and falls to him completely. Now loyal to the protagonist and his cock, she serves as his slutty onahole knight.

Thus, having the strongest spirit with big tits, princess knight Rufina, whom he can bang anytime and anywhere without any restrain, the protagonist's carefree and enjoyable life as the strongest master begins.

[Translated and edited from Getchu]