Dio, Arcane Harbinger

Flying, hexproof, vigilance, trample.
Whenever a Dragon you control attacks, it gains double strike until end of turn.

Whenever Dio, Arcane Harbinger attacks, add X mana in any combination of colors, where X is the total power of Dragon creatures on the battlefield. Spend this mana only to cast spells. Until end of turn, you don't lose this mana as steps and phases end.