Particularly in Commander, where the high bar set for aggro decks all but demands some kind of rapid multiplicative damage scaling, having all your creatures buff each other (for free!) is a true difference-maker
It is important to note that this is not a cast trigger, so if you have a way to blink your commander a few times or gain multiple combat steps, you can trigger this effect multiple times in a turn
Creature - Cleric Spirit Color tag templates(None) Shivan Devastator Flying, haste This creature enters with X +1/+1 counters on it
You see, when it or another Zombie enters the battlefield, all non-Zombie Creatures get -1/-1 until end of turn
It's also worth mentioning that since we are putting the card onto the battlefield, everything we play with Aid from the Cowl is uncounterable, which is very helpful against various control decks
I think this has some possibilities to be fun, but also I don't quite know if it's that impressive