But

*spoiler alert ch. 10*

We don't know yet if something happened to the blue king, it only shows his back, and Sage didn't have the marks until the following day... ┗( T﹏T )┛

i don’t remember them having sex.

Sorry, English translation are up to chapter 6, and as I pointed the sex scene is from "raw" chapter 10 (actually it's in Spanish, not in Japanese, but anyway) and since it's from the raw I added Spoiler at the beginning.

The conversation about two kings killing each other, or at least one of them dying, is in chapter 6, when Hink is talking to the Black king http://www.mangago.zone/read-manga/5_nin_no_ou/uu/m_chapter-6/pg-9/

I’m sorry every body I should have done the spoiler alert at the beginning of my question