If one of the two target creatures is an illegal target when Trial of Agony resolves, the remaining creature's controller must choose the remaining creature
9: You get an emblem with "Whenever a player casts a spell, you create a 2/1 blue Ninja creature token." Illustrated by: Evyn Fong Kaito, Cunning Infiltrator Historic Brawl decks Kaito, Cunning Infiltrator Brawl decks Rulings 2024-11-08 You don't have to choose a target for Kaito's first loyalty ability
Sneak Attack had a similar effect with some excellent reprints, most notably Oona, Queen of the Fae, Obelisk of Urd, and Notorious Throng
Syr Konrad, The Grim: Whenever a creature goes to the graveyard, Syr Konrad is spotting it and giving thin cuts to your opponents, in a way that supercharges Valgavoth