The way this works is that as blockers are first declared, the attacking player chooses the order that the blockers will be assigned damage later. Then once the trigger from Brimaz blocking resolves, that 1/1 cat gets inserted somewhere in that blocker order. But who chooses? The attacking player gets to make that decision!
So let’s look at a real-life example. You’re attacking with a Ghor-Clan Rampager that has a +1/+1 counter on it, and your opponent blocks with Brimaz and Soldier of the Pantheon. Realizing any damage to the Soldier will be prevented, you logically order the blockers with Brimaz first, then Soldier. After that, Brimaz’s trigger resolves and puts a 1/1 cat into combat blocking your Rampager. As the attacker, you can put it anywhere you want in the blocking order. I’d recommend putting it first or second, since putting it at the back will be pretty pointless in this situation!
Today’s Rules Tip written by James Arriola and Josh Stansfield