Collective Inferno (ECL 397)

Enchantment • Mana cost: {3}{R}{R} • Lorwyn Eclipsed • Mythic

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
As this enchantment enters, choose a creature type.
Double all damage that sources you control of the chosen type would deal.

Illustrated by YUE.