Chthonian Nightmare is only played in one specific deck that wins by taking an extra long turn, looping the same process of sacrificing and reanimating Priest of Gix repeatedly to generate infinite mana or infinite death triggers or both.

Not only is the loop nearly impossible to interact with, it forces the opponent to sit and wait while Chthonian Nightmare player plays solitare. It can even lead to the opponent timing out because, if they want to hold up interaction, they need to click "Resolve" 20+ times as the endless CT triggers go on the stack.

I don't feel the Chthonian Nightmare loop strategy is aligned with the spirit of the game, and so I feel it should be banned from both Historic and Timeless.