Black Zamasu is the immensely powerful result of the fusion between Black Goku and Zamasu by the use of Zamasu's earrings. Black Zamasu is regarded as the strongest being in the Future Timeline. Fused, he has the immense capabilities of Black Goku's Zenkai Boosts and Super Saiyan transformations, combined with Zamasu's immortality and technique. Black Zamasu has fought almighty opponents such as Gogeta, Lord Beerus and Zeno.
Black Zamasu can advance to Extended Super Saiyan Rose, which is way more versatile than the previous transformation, completely outclassing Goku, Vegeta and Trunks in their Super Saiyan Blue forms. As shown in his battle against Gogeta, Black Zamasu can manipulate the atmosphere, allowing him to pull forth attacks from other dimensions.
His base power alone was enough to defeat Goku, Vegeta and Trunks at their maximum power.
Gogeta is the only warrior that forced Black Zamasu to transform into an Extended Super Saiyan Rose.