When Construction Arsonist enters the battlefield, you may choose an instant or sorcery card in your hand. If you do, that card perpetually incorporates (Incorporate perpetually adds an additional casting cost to that card, adding any new colors from that cost, and adds abilities/text to that card for the rest of the game.) and gains "When you cast this spell, it deals 2 damage to each opponent." Whenever you cast a multicolored spell, Construction Arsonist gets +1/+1 until end of turn for each of that spell's colors.