Ashiok nightmare distributions seem more toward the bottom of deck
When playing Ashiok many times it seems that the last card in the opponents deck gets more than a random amount of nightmares. Things like 3 cards left and have 12 nightmares in their deck, the top two drain for 1 or 2 and the last one drains for 8 or 9. One reason might be how 'Get ahead' interacts with the bottom of the deck. If the nightmares on the bottom card simply move to the second bottom card then over the course of a game this could basically tripple the nightmares on the bottom card. (Assuming you cast 2 copies of 'get ahead')
I would suggest instead that the nightmares are either randomly redistributed across the rest of the deck, or that they trigger when you draw their bottom card (Although this might be too strong).