Elite Workshop Day 1 — What kind of item should we make?

Nightmare's Trail Wondrous Item (Boots), Rare (requires attunement) While wearing these boots, your unarmed strikes using your feet deal an additional 1d6 fire damage. You can use an action to increase your movement speed by 30 feet for this turn. When you do, your movement before the end of your turn leaves a trail of flame of up to 30 feet long. The flames last on the ground for 1 minute. A creature who starts its turn on the flames or enters its area for the first time on a turn takes 2d6 fire damage. You are immune to these flames. Once you use this feature, you can't use it again until the next dawn. Very Rare variant: Allows you to transport yourself and up to three creatures into the ethereal plane once per day. Whenever you do, roll a d20. On a 1, the boots lose the ability to do this

Dread Jouster Weapon (lance), rare This black lance has a spiral of molten, fire-red metal that winds its way down the length of the weapon. When pointing this weapon forward and moving, it expels waves of fire from its spiral. This weapon has a maximum of 4 charges and regains 1d4 at midnight. When you point this weapon towards a creature that is 15 to 60 feet away from you, you may expend a charge and leap forward, instantly summoning a Nightmare and mounting it, riding it to the creature’s location and making an attack with the lance that deals an additional 1d12 fire damage. The Nightmare remains until the beginning of your next turn and you maintain any benefits from being mounted on this Nightmare until dismounted or it disappears. Nightmare’s Carrot Ammunition (arrow), very rare This arrow is made of coal and ash, containing embers that glow with the desire to set something ablaze. When you make an attack against a creature that is on the ground with this ammunition, you may make a horrific whistle as you fire, which transforms into a Nightmare that follows the arrow as you fire. When the arrow reaches the target, the arrow explodes into ember filled ash, causing the Nightmare to make an attack with its Hooves against them and then disappears. During the Nightmare’s charge, any creature it passes within 5 feet of panics and must use its reaction to make an attack of opportunity against the Nightmare.

A necklace in the form of a horse shoe, it is always flickering with fiery embers and trails wispy ashes. When attuned, you can use it to cast Find Steed, calling a nightmare instead. This steed only last for a limited time, and can only bring its rider to the ethereal plane. CURSE. When attuned to this, you can not have a fly speed of any kind, just as the nightmare will never be able to fly again (since it was formerly a Pegasus).

Coat of the Nightmare Leather Armor, Very Rare(requires attunement by a non-good character) This long black leather coat is made from Pegasus hide and soaked with water from the River Styx. While wearing this coat you have a +2 bonus to your armor class and you gain resistance to fire damage and acid damage. In addition you are wreathed in flame and take on a nightmarish appearance that gives you advantage on intimidation checks and disadvantage on persuasion checks. Finally once per a day you may touch a non magical horse and use an action to turn the horse into a Nightmare under your control for 1 hour. Your DM will have the statistics for this creature. After you use this ability you must finish a long rest before you can use it again.

Nightmare Visage Wondrous Item, Very Rare (requires attunement) While wearing this item, you can use a bonus action to ignite your hair in flames (or make an image of hair made of flames if you dont have hair). This hair does not burn you. While this item is activated in this way, you shed bright light bright light in a 10-foot radius and dim light for an additional 10 feet, you have resistance to fire damage, and your melee attacks deal an additional fire damage. You can also cast the Produce Flame cantrip. Additionally, you can cast Etherealness once per day. The flames die out if you are unconscious. You can also dismiss the flames as a bonus action.

Fell-Rider Chariot Wondrous item, very rare A bronze chariot with iron reigns. As an action, you can crack the reigns and summon two bronze-armored nightmares to pull the chariot. The nightmares can be dismissed as a bonus action. The nightmares and the chariot's riders pulling the chariot gain a +1 bonus to their AC, and the chariot riders have resistance to fire damage. In addition, the nightmares can pull the chariot into the air and fly. If both nightmares are reduced to 0 hit points, they cannot be summoned again for 7 days.

Nightmare Facade Studded Leather Barding, Very Rare This barding was enchanted by a nightmare's flames. This barding grants a +1 bonus to its wearer's AC and saving throws. When you put this barding on a mount, flames run across the mount. These flames produce light but are harmless and do not burn creatures or objects near them. While wearing the barding, the mount sheds bright light in a 10-foot radius and dim light for an additional 10 feet. It also gains resistance to fire damage, and its melee attacks deal an additional 1d6 fire damage. A creature mounting the mount shares this fire resistance and additional fire damage. While mounted, a creature can also use its bonus action to change the color of the mount's flames.

A cursed bridle and tack set that once placed on a steed, it changes it into a Nightmare version of the creature. Yes that means someone could have a Nightmare Camel. The steed would need a Saving Throw to not turn evil

Maybe something with it's skin/pelt or maybe even something more akin to a graft from 3.5?

For Nalfeshnee, what about a set of panpipes made from its horns and decorated with its wing feathers, that can cause/inspire horror like it's nimbus ability?

The idea I have for the Nightmare is a whistle that will bring forth a Nightmare on command.