Nightmares are just the same as Dreamers accepted they use dark magic instead of light to form and make items.
The average of both species can use summoning magic to some degree, but they are not as versatile in their summoning capabilities as the Vaseterms are, and can only make a few things unless there specialty is in summoning.

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Nightmare are just like Dreamers when it comes to ones who are able to summon weapons, Dreamers use Light Magic to form and craft, clothing and weapons, these summoned items resemble the one who made them and Style is usually subconsciously done and no two Dreamers are a like mentally or by Hearts reflection, meaning there clothing and weapons will be unique to that individual. Now Dreamers usually do have a bright and elegant Style just like there true form as Nightmare is a dark and rigid Style.

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Just like how a Dreamer base element to form things is Light, a Nightmare is Darkness, both species can combined other elements ( if they have more than one) to there clothing and weapons, for example Malice will usually set his blade on fire or use electricity as Catty can only use fire, the light or dark magic works as a binding element to what ever the Dreamer or Nightmare alternate elements are.

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Dreamer and Nightmares are known for summoning a variety of different weapons mainly for humanoid forms.

These weapons can be anything from a variety of swords, daggers, Shields, Spears,halberd, Lance's, staffs, javelins, axes, hammers, mace, balls and chains/ribbons, wipes, Chakram, bows, crossbows,
There are even musical items that can be used for weapons as well and a lot, lot more.( Like a mop like what Purity uses)

Like stated previously a Dreamer and Nightmares are Limited on how many items they can summon, it's easier to make clothing then it is to make a weapon, and it's harder to make far range weapons.

All do some examples.

Malice can only summon a sword and bow, but he also has Darkness, fire and electricity, a long with more than one alternate form, he would be a above-average 'Balance' Nightmare ( because he special)

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Catch/Catty is what called a 'Summoner' he can form multiple items/weapons but lack in Elemental and alternate forms.

The average a Dreamer or Nightmare can usually summon is one to two weapons. The weapon is locked, meaning if the Dreamer is naturally a sword Summoner, then that is what they will summon they can't change there naturally Talent to a different weapon.the style of the weapon can change with the individuals age and mindset and this is the same for Nightmares.

Fun fact:
Malice sword has changed with the change in his personality.
( If you watch the playback, you can see both of Mal different sword side by side)

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Up above is just a few different styles of swords to different Nightmares and there tase in weapons and themes.

[Staying from left to right]

'Flaming red'
Here we have a Pyro sword, the individual who summons this focus more on combining the element of Fire with their blade. It Sharp but not as sharp as others blades, what makes it deadly is that it's extremely hot and can melt and burn what it comes in contact with.

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Malice's katana
(His sword actually has a name but I'll tell it in a different post)
A long sword made for one purpose, to deal as much damage to something if it doesn't kill it... Mal blade is not only one of the sharpest as it can cut through several men without effort, but the ridges on the blade are reverse acting like fish hooks so when his blade make contact it's not going to end will for the other person...his blade is also infused with corruption meaning he can infect others with it if he chooses.

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'Murder of Crows'
This is a very peculiar sword, as it's themed around crows which gives the impression so would be the Nightmare who summoned it, the style of sword is more of a fencing blade and is cloaked in Shadows.
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'Rebellion'
Cataclysms sword, the Nightmare version of Catch's twin blades, Catch's signature ribbon is replaced with a chain, the ones bright Blade is now made of Shadows and Darkness, the ridges along the sword is also the opposite of what Malice's sword goes.

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' Bane Fang'
This is a giant Greatsword, the Nightmare who summoned this weapon favors power over speed, it's more blunt then actually Sharp but that's how the Nightmare like it, crushing someone's bones is more painful and take longer to heal then a clean cut wound.

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'Disregard'
Shattered Dreams sword, the only male ancient Frost Nightmare, his blade is made to be a mockery of the Dreamers who stand to challenge him, his sword is wrapped with the bloody tarnished Dreamer ribbons, and the blade it's self is said to reflect the faces of it's last victims...

(ok that all I can think of, and if you have any questions about this topic just ask and I'll answer it as soon as I can)

Edit: I forgot to mention that, ancient inalfa nightmares can suppress their darkness and make their weapons appear to be more normal instead of nightmarish, Mal does this with his katana but not his bow

Ancient and Alpha* we're did it get that from? Xp

Epic! The last two look especially awesome! *o*

that last one is wrapped with the ribbons of slain dreamers?! oh my, that guy's metal... lol