Who is Efnisien from The Mabinogion and what is their importance?

Efnisien loves battle and strife, and his mutilation of the the Irish horses causes the war between Bran and Mathowlch. He is an enigmatic character, sometimes acting helpfully and sometimes causing needless, random harm and violence. He seems to personify the forces of death and chaos, and it is appropriate that he is the one who shatters the Cauldron of Rebirth.