Normally, combat damage is dealt all at the same time. In that case, Yarus, Roar of the Old Gods's last ability triggers once for each player that face-down creatures you control dealt combat damage to, regardless of how many creatures were dealing that damage. If any of those face-down creatures have double strike or first strike, the ability will trigger once for each player dealt damage in each combat damage step.
If a face-down creature dies but the permanent card it becomes in the graveyard leaves the graveyard before Yarus, Roar of the Old Gods's last ability resolves, it will not return to the battlefield.
If a permanent card that would be returned to the battlefield face down by Yarus, Roar of the Old Gods's last ability can't be turned face up for some reason, it'll still be returned to the battlefield, but it will stay face down. In that case, it will be a 2/2 creature with no name, mana cost, or creature types. Since it isn't disguised or cloaked, it won't have ward {2}.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Dmitry Burmak
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If Yarus, Roar of the Old Gods dies at the same time as one or more face-down creatures you control, its last ability triggers for each of those creatures.
