18 cards where the card types include “nightmare” and the set is “iko”

Boneyard Lurker (Ikoria: Lair of Behemoths #178)
Brokkos, Apex of Forever (Ikoria: Lair of Behemoths #179)
Cavern Whisperer (Ikoria: Lair of Behemoths #79)
Chittering Harvester (Ikoria: Lair of Behemoths #80)
Fiend Artisan (Ikoria: Lair of Behemoths #220)
Gloom Pangolin (Ikoria: Lair of Behemoths #89)
Grimdancer (Ikoria: Lair of Behemoths #90)
Helica Glider (Ikoria: Lair of Behemoths #15)
Hunted Nightmare (Ikoria: Lair of Behemoths #92)
Insatiable Hemophage (Ikoria: Lair of Behemoths #93)
Labyrinth Raptor (Ikoria: Lair of Behemoths #193)
Lurrus of the Dream-Den (Ikoria: Lair of Behemoths #226)
Necropanther (Ikoria: Lair of Behemoths #196)
Nethroi, Apex of Death (Ikoria: Lair of Behemoths #197)
Slitherwisp (Ikoria: Lair of Behemoths #208)
Snapdax, Apex of the Hunt (Ikoria: Lair of Behemoths #209)
Void Beckoner (Ikoria: Lair of Behemoths #104)
Zagoth Mamba (Ikoria: Lair of Behemoths #106)

Though this query has ended, its relics still slumber in New Argive.