When reaveled by Summons of Chaos does his cost is considered O or the fewest number of cards in any opponent's hand? The same with Call the Brayherd.

“The Letter X”
Unless specified by a card, card effect, or granted player choice, the letter “X” is always equal to zero.

X is specified by a card in this case, the Shaggoth Champion itself.

So no more 2-cost Necrodomo's Prophecy Shaggoth…

Thanks for super quick reply Mallumo, I guess this part of FAQ refers only to X-cost tactics played using Twin-tailed Comet or Slaanesh's Domination .