(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)

When Invasion of Karsus enters the battlefield, it deals 3 damage to each creature and each planeswalker.

//

Refraction Elemental

Ward—Pay 2 life.

Whenever you cast a spell, Refraction Elemental deals 2 damage to each opponent.

(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, it deals 3 damage to each creature and each planeswalker.

Ward—Pay 2 life.
Whenever you cast a spell, this creature deals 2 damage to each opponent.