Originally, Yawgmoth was a powerful wizard who became Phyrexia's god and master. Yawgmoth gave Phyrexian organisms a purpose: to thrive, to grow beyond the confines of Phyrexia and into the rest of the Multiverse. Over hundreds of years, Phyrexia evolved priests and acolytes who extolled and worshipped Yawgmoth, as well as demons and abominations of infinite variety to kill for him.
Yawgmoth orchestrated the invasion of Dominaria. Ultimately, he was defeated and destroyed, but his legacy remains.