He was the ruler of the original Zanarkand. A great summoner among summoners. He realized that Zanarkand was doomed when Bevelle invaded so he convinced his followers to become Fayth en masse so he could summon an idealized version of their city (DZ). Yu Yevon is protected by the armor of Sin, and has continued to summon continuously for 1000 years. It's by this point that the Fayth are growing tired of the dreaming.