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In the real world, a nightmare is a bad dream. In D&D it’s a bad pun that nevertheless became somewhat iconic. They’re only in the Monster Manual.
A nightmare looks like a horse, but is darker and edgier. Night Mare, get it? These creatures are native to the Shadowfell, and the only equine thing about them is their general shape.
Nightmares are intelligent, carnivorous, and evil. In the wild they roam in packs, like wolves, and have a taste for sapient flesh. They can be ridden and are particularly prized as mounts by powerful servants of evil. Getting a nightmare to agree to work as a mount involves defeating it in combat and making it choose between service and death. Therefore, having a nightmare of your own is a sign that you’re a badass in addition to being an edgelord.
Though they are native to the Shadowfell, their high resistance to fire and fiery appearance make me think they have something of Hell in them as well.
We only get a single Nightmare stat block: it’s a Large Shadow Magical Beast with the Mount keyword, and a Level 13 skirmisher with 138 HP. It has Darkvision and trained Perception, and both land and teleport speeds of 10. This does mean it can teleport at will as a move action.
Nightmares have 20 fire resistance, and get +2 to AC against opportunity attacks. Their mount trait is Hell’s Ride, which grants 20 fire resistance to the rider.
Nightmares fight in melee with their flame-wreathed hooves, which do a bit of physical damage and ongoing fire damage (save ends). Their special attack is Hooves of Hell (recharge 5-6), in which they run the speed and leave a 10-foot tall wall of fire in their wake. The wall lasts for a turn and deals 10 fire damage to anyone who enters it. Anyone who attacks the nightmare while it makes this move also takes the same damage.
I imagine a nightmare has little reason to stand still while Hooves of Hell is charged. It will play Fire Tron with the PCs for as long as it can, only stopping to kick someone while it waits for the power to recharge. And unlike a Tron bike it can run through its own walls just fine, even with a rider. A whole pack of them would be, well, a nightmare to fight for PCs who aren’t resistant to fire.
Sample Encounters and Final Impressions
There are two:
Level 13: 1 nightmare, 1 battle wight commander (the rider), and 6 battle wights.
Level 13: a medusa archer and a medusa warrior riding nightmares.
It looks like the mount rules give enemy riders and their mounts separate actions, so that wight and the medusas can still attack the PCs while their nightmare mounts run around setting the map on fire. When they provoke opportunity attacks, the PCs can choose whether to target the mount or the rider. In this case I would also have an opportunity attack against the rider cause those 10 fire damage.
Despite the somewhat punny name, nightmares are cool. An evil knight type should have an evil mount to go with it, and evil fire-horses are one of the classics (the other being some kind of skeleton or wyvern).