The concerned verse is from Atharva Veda 20.137.8.
To understand the context of the verse you need to read the previous and next verse. The concerned verse is highlighted in the context of the answer.
The black drop sank in Ansumati's bosom, advancing with ten
thousand round about it.Indra with might longed for it as it panted: the hero-hearted laid aside his weapons.
Saw the drop in the far distance moving, on the slope bank of Ansumati's river, Like a black cloud that sank into the water.
Heroes. I send you forth. Go, fight in battle.
And then the drop in Ansumati's bosom, splendid with light, assumed its proper body; And Indra with Brihaspati to aid him,
conquered the godless tribes that came against him.
Context:
Black drop - A fast moving Asura in black colour
reached river Anusmati with his ten thousand army. Indra who waited for the moment picked his weapons(7). Indra addressing Maruts and Brihaspati to fight against the black demon and his army(8). When Indra and his companions went to fight with the demon it assumed its proper body and later they killed him. Demon is potrayed as godless.