Ashiok, Nightmare Detain

{1}{u/b}{t}Target player exiles the top four cards from their library.
{2}{u/b}{u/b}Ashiok , Nightmare Detain deals one damage to another target creature. That creature gains a nightmare counter.
{4}{u/b}{u/b}{t}Gain control of all creatures that have a nightmare counter for as long as I control Ashiok , Nightmare Detain. Put a 1/1 Counter for each creature I control with a nightmare counter.