Himitsu – Kuro no Chikai - Chapter 2, Part 4 – Contract

The world was once at war with 10 alternate worlds. This was known as the Concept War and our world won 60 years ago. All of this was concealed so no normal people are aware of this fact in the current time.

After the death of his grandfather, the high-school student Sayama Mikoto is suddenly summoned by the giant corporation IAI. He learns this world is headed for destruction due to the acceleration of the Minus Concepts. To prevent this, negotiations must be undertaken with the survivors of the other parallel worlds so their 10 concepts can be released.

And so Sayama is caught up in the final battle to clean up after the Concept War which left behind so much enmity.

You are the ruler, I am a subject, you want loyalty, I’ll give you loyalty. You are the ruler, I am the subject, you want my sacrifice, I will sacrifice for you. This lifetime, we’ll only be ruler and subject…… cannot become the one who accompanies you to experience everything.

A deposed empress, a failure of a substitute. Charging into battle, he was the emperor, and she, his general. But within the intimacy of their military tent, he was her husband, and she, his wife.

But everything was star-crossed. For the one he loved the most, he would push her into hell again and again.

- Description from Novelupdates

Protagonist travels between different anime worlds to right wrongs and uncover his reason for being along the way.

A disappearing gold mine. An old family secret. An army officer carrying a grudge spanning two generations.

As a small time reporter at the local newspaper, Shen Yu already had enough on her plate. Biased managers, check. Long, brutal work hours, check. The department beauty snatching her big story opportunity, check. But now another problem has reared its head and it comes packaged in a young and attractive military officer. He is on the hunt for some legendary space gold and he will get her to help him, whether she likes it or not.

Secrets are abound and Shen Yu soon finds that not everything is not as simple as it seems. Even her own past becomes questionable the more time she spends on the quest. Events from decades ago are rising up to haunt her and she needs to find out the truth before it’s too late. If only she had a better partner.