So this guy invites me to 8 games, all identical to each other, at the same time. What the heck?!

Blacklist
It may be rigged.

It's a tournament that I don't remember ever seeing.

Probably a Poonsquad tournament

Oh, those are round robin. It is very simple. You join and regret it later after a few games. RR are best with very low amount of players. You also need to account that a few players will lose a lot and won't want to finish all rounds. Unless it is equal skill.

Yup I see that now. Oh well.