`yum history undo $everything` to bring CentOS back to original state?

I made a few yum transactions (8 to be exact), and now my CentOS 7 is messed up. Is there a way to undo everything in one command?

You can use yum history undo to undo those commands serially, in the reverse order they were "done".

It's not clear what you mean by "messed up". Note that yum history undo does not roll back at the filesystem level. It is not like a "windows restore point". yum history undo just tries to, erm, do the opposite of what it did initially, in the hope that this will result in returning the system to a prior state of workingness. If your system is actually "messed up", it is unlikely to do much good. You should just start over with a fresh install.