His two attackers are flanking the brown treant so the pink pollen is coming from the right and the carpet of thorny vines is attacking him from the left. He ran forward to meet them. This is his domain so he knows what he is dealing with. He knows exactly how powerful the attacks are and what the concepts of the origin gods are. So he wasn't surprised by what happened next.
The black beam in his mouth exploded outwards. It wasn't a real explosion. It is more of a scattering or dispersion. He was forcing the small beads of devouring into the form of a beam. Now he stopped holding them together so they scattered into the environment and began to wreak havoc.
His three opponents did not like that change. They turned to defend themselves which turned out to be a bad idea. The black beam is actually made from the law of devouring, slaughter, and blood. These three laws each are stronger or equal to the law of life that their concepts are made of.