So Agatha's ability won't let you reduce an ability below 1 mana, but that's including colored mana, right? So an ability that was 4R could be reduced by 4 to just R since it's still one mana.
Tbh, I have been brewing around him so much, i just didn't want to put him in every gruul deck I make. But it makes sense with the activated ability. He is switched in the decklist!
