You cannot attack that player with the Thirster of Blood this turn, but in a multiplayer game you can attack, and then if you slip through another untap, another extra combat and another attack
I think the best thing about the format is that no matter what happens we have access to the vast majority of the cards (or all of them if your group is into it) and that since our decks are eternal we can just build as we go
This is a powerhouse of awesomeness and it's just a terrifically strong effect to toss on out there
Rather, a second "reflexive" ability triggers when you put a +1/+1 counter on Etched Slith this way
There are lots of great preconstructed Commander decks out there already that are fun and a good way to learn the Commander format, and you can get many of them for under 40
There is an end of combat in which you can activate this, right after combat damage