With Olivia distracted, Sigarda tore free from her bindings and summoned her Host of Herons.

Sigarda's Summons

Creatures you control with +1/+1 counters on them have base power and toughness 4/4, have flying, and are Angels in addition to their other types.

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Removing all +1/+1 counters from a creature affected by this ability or removing Sigarda's Summons from the battlefield will cause that creature to revert back to what it was. Notably, this may cause a creature to lose flying after it has been declared as an attacker but before blockers are declared. This may also result in a creature with damage already marked on it being destroyed if that damage is greater than the creature's toughness.