The process of enchantment that leads to the creation of the related magic of flame and dusk tongue blades is unique in the making of items, in that various intentional or accidental factors can lead to fluctuations in the power of the end result..  In the case of Midnight Rose (who sometimes also calls herself Midnight Thorn, depending on her mood), this blade was very deliberately worked by a faerie smith who followed the Witchlight Carnival for the variety of professional challenges; and who was commissioned by Phipnaxi Reboonsmest Leewamba's adoptive faerie parents to forge a trio of faerie treasures for their daughter to wield the inevitable day her wandering gnome spirit called her back to the mortal world.  As such, this sword is a unique treasure designed to grow with her needs.  The two rapiers, being more powerful, are guarded with a faerie glamour; that only she can wield the blade without it blazing up in intolerably bright flame or intolerably vacant and hungry darkness and blinding the user, and that she must prove herself worthy of them before they act for her as anything other than an ordinary rapier.  The third treasure, a moon sickle +1, was given her to wield until then and to guide her on the path of the fey wanderer.  As such, its full magick is available to her immediately.  The powers of the rapiers are earned by her making a name for herself and doing deeds that would make her foster fae parents proud; gaining experience levels is a good enough analogue, and allows DMs of any campaign that Phipnaxi may find herself in during her wanderings across the multiverse to tailor the powers of her blades which are available to her at that moment to the average level of magic item which other characters her level possess; as a generic guideline, absent a specific campaign, follow the following table:

• lv. 2: +1 to melee attack and damage
• lv. 3: fire / dark "flame" illumination/darkness effects, including effect of blending them
• lv. 5: fire / dark "flame" actually deals fire / exhaustion damage
• lv. 7: faerie glamour power

MOONLIGHT ROSE'S DESCRIPTION FOLLOWS:

 

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Once Phipnaxi proves herself worthy to wield it at 2nd level, Fireflies' Kiss will always be a magic rapier for her with +1 to attack and damage. In addition, after she reaches 3rd level she can use a bonus action to speak it's command word, causing flame-shaped wisps of shadow to erupt from the blade. These 'flames' shed darkness in a 40-foot radius and an equal radius of shadows (causing vision to be lightly obstructed) for 40 feet beyond that. At Phipnaxi's 5th level and beyond, the blade's 'flames' of shadow spread to any target it hits. These 'flames' do no additional physical damage to the target. but they continue to 'burn' the victim on subsequent rounds, draining their life force and causing them to gain a level of exhaustion each round (after the first) they are psychically 'burned'.  This effect can be resisted if the creature succeeds on a CON saving throw; and if they do so twice consecutively, the flames cease to burn them. The DC of this save is 15 the first round, and drops by -3 each round thereafter.  The blade continues to be sheathed in 'flames' of shadow until you use a bonus action to speak the command word again or until you drop or sheathe the sword.

If Phipnaxi is wielding both Midnight Rose and Fireflies' Kiss after she reaches 3rd level. and ignites them both at the same time so that any part of their areas of light or darkness/shadow overlap, roll 1d12 and consult the following table to determine the effect:

• 1 - 2: Flame tongue's light is extinguished; shade tongue's darkness/shadow takes full effect 
• 3 - 6: Flame tongue's light is extinguished; shade tongue's area of darkness/shadow is halved
• 7 - 9: Shade tongue's darkness/shadow are dispelled; flame tongue's area of light is halved
• 10 - 11: Shade tongue's darkness/shadow is dispelled; flame tongue's light takes full effect
• 12: An area equivalent to an 80' diameter sphere centered on the point equidistant between the rapiers is filled with a mist of writhing, constantly flickering sparks and shadows that confound the vision of all beholders.  All within this area, including the wielder and allies, must make an INT saving throw at DC 13 or be mesmerized by the pattern and behave as if they were under the influence of a confusion spell, with the following changes: on a roll of 2-6 the creature is affected as if an Otto's irresistible dance spell had been cast upon them; and on a roll of 9 or 10 they are afflicted with short term madness for 1d8 rounds but are subsequently unaffected by the interplay of light and shadow in the area and can see through it as if it were illuminated by normal daylight (to all others the area is moderately obscured, acting as a lightly obscured area to them and additionally treating attacks they make or spells they cast against creatures in this area as if those creatures had half cover)

After 7th level, provided Phipnaxi has Fireflies's Kiss somewhere on her person, she can call on the full faerie glamour power of Midnight Rose: An area equivalent to an 80' diameter sphere centered on the point equidistant between the rapiers is filled with a mist of writhing, constantly flickering sparks and shadows that confound the vision of all beholders, very similar to the last mentioned effect above; however, the shadows in this mist predominate, oozing and melting in a bewildering kaleidoscope of multihued shadow tones; and the whole area is permeated with the smells of licorice and opium poppies and filled with the sounds of eldritch bagpipes.  All within this area, initially including the wielder and allies, must make a CHA saving throw at DC 13 or be overwhelmed by the faerie personality of the sword, which will draw them into dreams of darkness and gloom, corruption and vapor and despair (roll d6: the character is the target of, affected by, or has centered upon them a(n)  {1: stinking cloud, 2: darkness, 3: [spell[fog cloud[/spell], 4: gaseous form, 5: infestation, 6: putrefy food and drink[} effect. 

  If Phipnaxi does not wish this to happen, she can make an attempt to contest the sword's personality in order to prevent the effect from touching herself and 1d8 companions.

Sentience. Midnight Rose is a sentient chaotic good weapon with an Intelligence of 13, a Wisdom of 10, and a Charisma of 13. It has hearing and darkvision out to a range of 120 feet.

The weapon communicates by transmitting emotions, sending a tingling sensation through the wielder’s hand when it wants to communicate something it has sensed. It can communicate more explicitly, through visions or dreams, when the wielder is either in a trance or asleep.

Personality. Midbnight Rose seeks to protect Phipnaxi;  it also seeks the increase of wonder and joy in the world; and the advancement of the Feywild and of the Seelie Court and creatures within it. Courage, loyalty, beauty, music, and life are all part of this purpose, as the ancient elven ideals that moonblades reflect all had their roots in the Feywild.

If Midnight Rose has a flaw, it is its impatience with details and nihilistic personality.  Cheerfully nihilistic, but still ... 

Proficiency with a Rapier allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Melee Weapon Attack: Piercing, Damage: Piercing, Exhaustion, Damage, Combat, Finesse, Vex