Pack Tactics

Beastmaster Approach: 3rd level When you have advantage on an attack roll against a creature your companion has dealt damage since the start of your last turn, you can reroll one of the dice once. When your companion has advantage on an attack roll against a creature you've dealt damage since the start of your last turn, it can reroll one of the dice once.

Granted by Beastmaster Approach (Companion) at 3rd level.