Relentless Onslaught

When your creatures attack for the first time each turn, if creatures you control have total power fifthteen or greater, untap all creatures you control. After this phase, there is an additional combat phase which creatures you control have trample until end of turn.
Whenever a creature you control attacks, it gets +1/+1 until end of turn. If there's another combat phase, double that effect for each additional combat phase.