You and another bard with the Harmonious Duet feature have learned to harness harmony and discord to reinforce each other's musical and combat techniques.
Call and response: While in the presence of your duet partner, you can Countercharm as a bonus action. Your ability to maintain it is also enhanced; if one bard begins to Countercharm and is interrupted or threatened, their duet partner may use their reaction to take over the Countercharm and use their bonus action to continue it.
Harmonize: When one bard grants Bardic Inspiration, their duet partner may use their reaction to harmonize with the performance and grant an extra d6 to the inspiration roll.
Yeah, Boss!: When one bard casts Vicious Mockery, the other may reinforce the insult in an irritatingly nasal voice. This has no impact on the effect of the Vicious Mockery or anything else, but it cannot be silenced in any way.
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