All permanents on the battlefield (Sakashima of a Thousand Faces as a copy of Havi).
Steps
Activate Yahenni by sacrificing Sakashima of a Thousand Faces.
When Sakashima of a Thousand Faces dies, it and Havi trigger.
Resolve the Havi trigger, returning Sakashima of a Thousand Faces from your graveyard to the battlefield as a copy of Havi.
Resolve the Sakashima of a Thousand Faces trigger, returning any legendary creature card with mana value 5 or less from your graveyard to the battlefield.
Resolve the Yahenni ability.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite sacrifice triggers.
Infinite death triggers.
Put all legendary creature cards with mana value 5 or less from your graveyard onto the battlefield.