Boraxis Megatheros is an ancient giant who corrodes anything living in his path. He was sealed away at some point, likely 1210, in The Garden of Spires by The Order of the Spoken Flame. Liam was charged with guarding his prison, and did so without incident for nearly 800 years, until Dr. Sugarman summoned the spire containing him back into the real world. Once he was free, Boraxis began to wreak indiscriminate havok upon Dakota. He was defeated by Xombi and Catholic Girl, and resealed by Julian Parker and Rabbi Sinnowitz.
Every footstep he takes releases deadly toxins into the earth, and his very breath chokes the air with poison. Everything he passes dies, and everything he touches decays. It is difficult to damage him, and he can recover quite quickly by drawing pollution back into his body.
Boraxis Megatheros is also a surprisingly deep metaphor. While he is an entity of pollution, he was sealed away hundreds of years before modern pollution really existed. Dialogue references him wishing to return the planet to a super-primal state of non life. Thus he represents forces both old and new simultaneously.