After you best Cyrus for the last time you never see him again. The last place you ever see him is the distoration world. Did he choose to stay in a world with nothing in it if he couldn't make a perfect one?
After his defeat at Spear Pillar, Cyrus claims he will have revenge and disappears. His assets and duties are left to Saturn.
Source:Bulbapedia
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When you defeat Cyrus in the Disortion world, he never stays in that world, he just lives off as a normal person, but his dream never came true. (Hope This Helps!)
