You gain a +1 bonus to attack and damage rolls made with this magic weapon.
This segmented longsword has Infernal runes inscribed into the metal, reading 'release'. When the command word is spoken it unbuckles into a deadly chain whip. You may use a bonus action to change the weapon's form between a longsword and a whip. If you are proficient in 2 or more martial weapons, 1 of which must be longsword or whip, then you may use your interaction to change form the same way you would draw a weapon.
Longsword. 1d8 slashing, versatile (1d10), transforming
Whip. 1d6 slashing, reach (10ft), finesse, transforming
Whilst in whip form you may choose to attack with disadvantage. If you do so, you may hit three creatures within 5ft of each other and within your reach, with the same attack. This attack does 1d4 slashing damage to each target it hits.
Morph Attack. You may your bonus action or interaction (if you meet the requirements) to transform this weapon in between two attacks. On the second attack you may choose to attack with disadvantage. If the attack hits, a creature is incapacitated until the end of its next turn
Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.
Notes: Bonus: Magic, Speaking the words 'Release' whilst holding the weapon, Damage, Combat, Versatile
So, the whip from Bloodborne. Neat
Yup, tried my hand at making a trick weapon. Been a while though so it needs some serious refinement so it's not so wordy and complex