Mona, Spirit of Wisdom

Flying, lifelink, legion (For each creature with
legion, creatures you control that share a type with a
legion gain another legion level:
I. Creatures of this type cost you {1} less to cast.
II. Creatures of this type you control have +1/+1.
III. Whenever a creature of this type enters the battlefield under your control, draw a card.)

As long as you control three or more creatures
with legion, other creatures you control have indestructible and lifelink.