The Barems were created as a byproduct when Dagahra absorbed pollution. They were even more toxic than the pollution itself, and were what killed the Atlanteans. When Dagahra was re-awakened, he began producing more Barems.
They caused destruction when they washed up on shore, and they were used as a weapon by Dagahra to fight off Mothra Leo. Mothra Leo ultimately destroyed the Barems after he split into many miniature versions of himself and destroyed them from the inside out, causing Dagahra to self-destruct.