Activate Altar of Dementia by sacrificing Triumph of Saint Katherine.
Triumph of Saint Katherine dies, triggering itself and Verdant Succession.
Resolve the Triumph of Saint Katherine trigger, exiling it and the top six cards of your library into a face-down pile, then shuffling that pile and putting it on top of your library.
Resolve the Verdant Succession trigger, searching your library for Triumph of Saint Katherine and putting it onto the battlefield.
Resolve the Altar of Dementia ability from step 1, causing target player to mill cards equal to Triumph of Saint Katherine's power.