When Terisian Mindbreaker’s attack trigger resolves it instructs the defending player to mill half their library, rounded up. If Bruvac the Grandiloquent is on the battlefield and the defending player is an opponent of Bruvac’s controller, Bruvac’s replacement effect doubles the number of cards milled, so the defending player mills twice the amount (2 × ceil(library_size/2)). See , , , and Ruling [2020-06-23].
Step 1
Declare attackers; Terisian Mindbreaker becomes attacking and its triggered ability (“Whenever this creature attacks, defending player mills half their library, rounded up”) triggers and is put on the stack.
Step 2
When the triggered ability resolves it determines the number of cards to be milled: compute N = half the defending player’s library, rounded up (using the library size at the time the ability resolves).
Step 3
Bruvac the Grandiloquent is a static replacement effect that applies to any event where an opponent would mill one or more cards. Because the resolving ability is instructing the defending player (an opponent of Bruvac’s controller under our assumptions) to mill N cards, that is an event that 'would mill' N cards and Bruvac replaces it with 'mill twice that many cards instead.' See and Ruling [2020-06-23].
Step 4
The replacement effect makes the event 'mill 2 × N cards.' The defending player then puts the top 2 × N cards of their library into their graveyard, or if fewer than 2 × N cards remain, they put all remaining cards into their graveyard.
Step 5
If the defending player’s library becomes empty because of that milling, they don’t lose the game from milling itself; loss by drawing from an empty library only happens if they later attempt to draw a card as described in .
Resolution
Terisian’s triggered ability calculates N = half the defending player’s library, rounded up, then Bruvac’s replacement effect changes that event so the defending player mills 2 × N cards instead (or the remainder of their library if fewer cards remain). If Bruvac isn’t on the battlefield or the defending player is not an opponent of Bruvac’s controller, Bruvac doesn’t apply. See , , , and Ruling [2020-06-23].
