All permanents on the battlefield (Glasspool Mimic // Glasspool Shore as a copy of Cavalier of Night).
Steps
Activate Yahenni, Undying Partisan by sacrificing Glasspool Mimic.
When Glasspool Mimic dies, its last ability from Cavalier of Night triggers, returning itself from your graveyard to the battlefield as a copy of Cavalier of Night.
When Glasspool Mimic enters the battlefield, its first triggered ability from Cavalier of Night triggers; choose not to sacrifice a creature.
Resolve the Yahenni, Undying Partisan ability from step 1.