A classic reprint from Kamigawa block
Rat Colony is very easy to cast, letting you flood the battlefield with Rat Colony copies easily, especially in the late game when you have a lot of mana to work with
Renown creatures are good and important, and they make blocking hard once they get through, and there are a lot of ways to push damage
As long as the card is the chosen color, and either one color or two colors and blue, it can be in your deck regardless of color identity
It fits the character perfectly, it balances the cards power level and casting costs pretty well, and it can ultimately be broken in really absurd ways
Now that most multiplayer games begin with 40 life in Commander, Congregate is less played, but its power and class should be respected