The fourth 3-in-1 edition of X (aka X/1999) contains volumes 10, 11 and 12.
It is rated T+ for older teens (16+). Another warning that X is quite graphic. CLAMP are able to draw very detailed internal organs being splattered against the wall. I wouldn't want to see the types of reference materials they used for X, unless they didn't use any, then that's really talented!
The first part of this volume feels a bit like they are throwing a lot of information at you. All good information though! It answers a lot of questions but the story is still slightly confusing. It's very text-heavy.
It's nice to see a bit of Kamui's school life too. With everything that is happening, it's surprising that he is still able to even think about education!
The final part is mostly about my favourite character, Subaru. Including a short story focused on him! I've mentioned in a previous review about my love for Tokyo Babylon, the manga that Subaru is originally from. So check that out too if you're interested!
This isn't my first time reading X, I first read it quite a few years ago when I was battling an eating disorder and depression but one of the lines from X really helped me through that:
"When someone chooses to eat, it's the same as saying they're choosing to live."
I adore that line. Powerful quotes like that is why this manga will always have a big place in my heart.