While phb will contribute to turn 1 kos, I don't think the card in particular will contribute to it that much. What cards in our format are donkable? Squirtle, tynamo, deino, sableye, emolga, piplup. 70 hp max? If we use rayquaza we would need a rayquaza start, a lightning energy, a phb, and a gym. We already have tornadus that only needs a tornadus start with dce and any stadium. In terms of donking its actually more difflcult to go the phb route. You do gain +10 damage, but when you comapre that to the less chance of attacking turn 1, and the fact that any of these donking basics are much worse late game than our tornadus ex, this donk deck concept is worse than donk cards we already have.
Of course you can run phb with virbank in a tornadus donk deck, but the damage goes from 60 to 90. This will only really donk the 70 hp pokemon. We can already do that with pluspower. We can already donk out the wazoo, but it honestly isn't a huge deal. I doubt this card by itself will turn things into any sort of a donk fest.
The card is very strong. With keldeo or switches the effect of the card can be limited. +30 damage is nice, but how many 3 hkos become 2hkos, and how many 2hkos will become 1hkos? Most our attackers 2hko all other attackers, with or without this combo. However I'm sure the damage will naturally allow for amazing darkrai 90 + 30 plays. It will also allow darkrai to ko non ex attackers much more effectively. Add dark claw and the damage reaches 140 + 30. This twice will ko 2 exes in two attacks. That is ridiculously fast. This also kos terrakion in one attack. Phb in general will also make retaliate much more difficult to pull off, considering if poison kos a pokemon, retaliate doesn't trigger.
I feel darkrai gets the most out of this card. I can't see a deck like keldeo blastoise running any, though. Those 6 slots for phb and virbank could be 6 energy, each worth +20 damage.
I'm also sceptical of a hooh deck running the card. I feel like darkrai can take advabtage of the combo so much more effectively, that tornadus might be better off countering virbank for 20 more health, and reducing the poison damage. This way tornadus can still deal with darkrai on somewhat even footing.
Landorus can be ridiculous with this card as well. Assuming you can use the combo twice, you can 2hko darkrai with 1 energy. While setting up the bench with 60 damage. This will likely give straight darkrai a bit of trouble.
