Nightmare Dungeons have no level requirement, we just give you a level recommendation.

Nightmare Glastheim 1 and 2

Speak to Hugin's follower (glast_01 196, 273) near the OGH Instance NPCs and the Kafra.
Tell her That is interesting. please tell me more followed by I really want to do that!.

Choose between:
Monastery Cemetery 5,000z (Level 100+)	Corridor of 2nd Floor 10,000z (Level 140+)
Grave Keeper with Hatre (Wraith) Wraith Dead Elusive Wraith Dead Corrupted Priest Cursed Box Dark Lord (MVP)	Wandering Man Cursed Book Cursed Box Corrupted Priest Alice Chimera Mysteltainn Nightmare Baphomet (MVP)
Cards
Monastery Cemetery	Corridor of 2nd Floor
Wraith Dead Card (4189) [MVP] Dark Lord Card (4168)	Alice Card (4253) Mysteltainn Card (4207) Chimera Card (4300) [MVP] Baphomet Card (4147)

Pyramids

Use our Warper, Dungeons and warp to Pyramid.
Go to the center of the map and enter the warp.
Walk to the south and talk with the Suspicious Cat. (moc_prydb1 103, 54)
Inside the dungeon, you can get two hunting quests from the Tomb-Raiding Cat.
Talk to the Cat to obtain repeatable quests to kill either 20 Verits or 20 Ancient Mummy.

Floor 1: 5,000z (Level 100+)
Nightmare Minorous Nightmare Verit Nightmare Mummy	Nightmare Minorous Card (4655) Nightmare Verit Card (4658) Nightmare Mummy Card (4656)
Floor 2: 5,000z (Level 100+)
Nightmare Mimic Nightmare Arclouse Nightmare Ancient Mummy Nightmare Amon Ra (MVP)	Nightmare Mimic Card (4654) Nightmare Arclouse Card (4653) Nightmare Ancient Mummy Card (4657) [MVP] Nightmare Amon Ra Card (4652)

Right before 2nd floor portal, Belljamin Button (c_tower1 232, 222) will teleport you to Nightmare Version of Clock Tower.
You need to be at least level 150 and will be charged 10,000 zeny for each entering attempt.
It has a monster range from 155 (Neo Punk) to 170 (Owl Marquess). It has also a unique MVP wandering in 3rd floor of Nightmare area called Time Holder.

Monsters	Cards
Cursed Box Cursed Book Neo-punk Big Ben Timekeeper (MVP) Arc Elder Owl Viscount	Neo Punk Card (4628) Big Ben Card (4626) Time Keeper Card (4630) Arc Elder Card (4629) Owl Viscount Card (4631)