All permanents on the battlefield (Master of the Veil face down).
Steps
Activate Yahenni by sacrificing Master of the Veil.
When Master of the Veil dies, Yarus' second ability triggers, returning Master of the Veil from your graveyard to the battlefield face down, then turning it face up.
Master of the Veil triggers, turning itself face down.
Resolve the Yahenni ability, giving it indestructible until end of turn.