I actually have to homebrew it for my bladesinger who choose shortsword proficiency at lv. 2, the sword does not correctly display the attack bonus...

Yeah pretty much 

@RevRawr Sun Blade “It is a shortsword that does longsword damage” is incorrect:

It has the finesse, but not the light property of a shortsword, deals versatile 1D8 one-handed and 1D10 two-handed radiant damage, with an additional 1D8 for undead targets, and you add your Strength or Dexterity modifier +2 from the sword to the attack and damage rolls, and also your PB if you are proficient with shortswords or longswords. A monk has shortsword proficiency to do this too. Remind that versatile is not the two-handed property, compare quarterstaff for a monk.

So e.g. a rogue, who has swapped its non-finesse longsword proficiency to a finesse and reach whip (by using TCoE p7), would still be proficient with the sun blade by its proficiency with shortswords, using the sun blade one (1D8 damage) or two-handed (1D10 damage), and can add its Sneak Attack bonus when fulfilling its requirements.

For that the hilt only has a weight, proficiency with any sword weapon (compare luck blade: greatsword, longsword, rapier, scimitar, shortsword) could count for a sun blade—but nothing really fits; in fact for a ‘real’ lightsaber rapier would be the best fighting style to use it and two-handed use makes no sense, maybe for kenjutsu. Most wild wielders trained with iron blades would stab their own backs, not considering the blade's length and weightless form.

Otherwise the activated blade can be ruled by a DM as being invisible with the bearer of Invisibility or Greater Invisibility, but it would be much more fun, to give the activated sun blade the humming ability of a lightsaber. The description should add this: “When using a form of invisibility that makes the equipment invisible too, the hilt is silent and invisible with the bearer, but the active luminous blade and its emitted light is not. In addition the active blade gives a distinctive humming sound, particularly when swung towards an enemy. You can't profit from the condition of being invisible, when the opponent is not blinded and deafened while the sun blade is activated.“ instead of the only in dndbeyond existing, confusing “Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.”

The noise seems pretty useful to limit this item from OP. Don't forget to combine such a bright and noisy weapon with a good portion of Booming Blade's thunder damage or Green-Flame Blade's fire damage!

Much more interesting than Invisibility is the question about a warlock with Devil's Sight using the Darkness spell—which level of darkness can be dispelled by a sun blade's light … none, 2nd level, 3rd etc.? Maybe deal the emitted light as cast by the cantrip Light or the 2nd level spell Flame Blade?

Can Dispel Magic deactivate an opponent's sun blade? Control Flames or Prestidigitation can't do so, definitely.

P.S.: The only frustrated monk should be the shadow stepper, along with the gloom stalking ranger or a shadow walking sorcerer, just for wasting a bonus action before attacking with a still drawn weapon. But be cool, activating a magic item is almost a full action—be happy with the bonus action!

Me: "If only 5e had some sort of magical lightsaber that I could give my Bladesinger to complete the Jedi aesthetic..."

Sun Blade: "My people need me."

for your questions yes the sunblade works as a finesse weapon even when twohanded. tried it out with one of my rouges. it actually is about the best weapon a rouge can get access to. 

Quote from SigourneyCleaver >>

@dndbeyond, is there any way you can count this as a shortsword and a longsword in your items? Trying to equip this to my monk that is only proficient with shortswords and I cannot override the proficiency. I'm having to go in and manually override the -to hit bonus, etc every time I open the character page and its a pain.

 You can use it as you have shortsword proficiency but it won't count as a monk weapon for certain abilities like the bonus action attack you get.

Also you can now play a race or multi-class into a class that has longsword proficiency like elf or fighter & use Dedicated Weapon to make it a monk weapon!

Question, What's going to happen if you press the hilt against someone's jaw and activate the blade as a bonus action? Would the blade spring into existence and went through their skull?

Or could you simply break a lock/stuff by using the same method?

can you have different colored sunblade

My DM gave me one that is a "Druid sun bade" and it looks like a stick of wood until activated- so it's not lit by default.  But this is a house rule.

Quote from JavaPhysics >>

Question: This item seems to have two somewhat contradictory statements in it:
(1) "If you are proficient with shortswords or longswords, you are proficient with the sun blade."

and

(2) "Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it."

Doesn't statement #1 ALONE IMPLY that you would add your proficiency score to the attack roll if you were PROFICIENT with either a SHORTSWORD or a LONGSWORD? Then why add statement #2 at all? and... more importantly, why does statement #2 partially CONTRADICT statement #1

So.... If you are "proficient with a shortsword" are you proficient with sun blade or not?

AND.... IT'S A BIG DEAL.... because.....while we are on that topic... It further states that "this magic longsword has the finesse property" ! Sooo..... does this imply that a character who normally only uses SHORTSWORDS with DEXTERITY (not strength) can now suddenly use this LONGSWORD as a LONGSWORD, but using their DEXTERITY for the attack, taking advantage of a longsword's 1d8 slashing damage one handed.... and 1d10 slashing damage TWO HANDED!?? And on top of that you add +2 to hit AND +2 damage? (Plus 1d8 damage on top of that to Undead!?)   THAT is GREAT for someone who normally USES a LONGSWORD (with strength) but CRAZY GREAT weapon for someone who normally CAN'T use a longsword, but only uses short sword.... Um.... Imagine taking away a dexterity-users shortsword that normally does 1d6+dex damage, and replacing it with a two-handed longsword that can do 1d10+dex+2+1d8 to undead..... You went from an average damage of 3.5 + dex.... to 7.5 + dex (+4.5 to undead!)....

- - - - - - - - - - - - -

Um. OK. Nevermind. I just now realized that EVERY CLASS that is PROFICIENT with a short sword is ALSO proficient with a LONG SWORD except the MONK...... And, in that case, the Longsword would still NOT be counted as a "monk" weapon.... So using this great, shortsword/as a longsword would NOT be practical for a Monk.... So its a moot point. (I'll leave my comment here anyhow.... Maybe someone will correct the typo in the DM Guide or something.... ha.  Or maybe someone reading this has a Monk character, or is a DM with a Monk character and wants to read my ravings on this topic.)

 It's not actually a typo. In dnd beyond, the phrase "proficiency with a [weapon name] allows you to add your proficiency bonus to the attack roll for any attack you make with it." Is automatically copy and pasted onto the bottom of any magic item. 

My Bard/Sorcerer got one of these on a lucky roll of a trader that sells random magic items. I wish it wasn't a bonus action to activate, I lose a bonus action in combat since if I had it active while sneaking it would just give me away, but I understand why 

Quote from Faerik >>

"Much more interesting than Invisibility is the question about a warlock with Devil's Sight using the Darkness spell—which level of darkness can be dispelled by a sun blade's light … none, 2nd level, 3rd etc.? Maybe deal the emitted light as cast by the cantrip Light or the 2nd level spell Flame Blade?

P.S.: The only frustrated monk should be the shadow stepper, along with the gloom stalking ranger or a shadow walking sorcerer, just for wasting a bonus action before attacking with a still drawn weapon. But be cool, activating a magic item is almost a full action—be happy with the bonus action!"

100% this. The description does say "The light is sunlight". Is sunlight magical? I would assume not but I'm not sure. Darkness spell cannot be illuminated by non magical light.

Now if this is the case, the smallest light radius you can make the Sun Blade is 20 feet and Darkness is 15 feet. Would that make a ring of light around the perimeter or would the Darkness snuff it out?

May not be that bad for a shadow stepping monk with Darkness cast on the Sun Blade (with the ability to see in magical darkness of course). A couple of levels into Warlock gets you Hexblade for the longsword proficiency (if you don't believe shortsword proficiency is sufficient- which it should be) and Devil's Sight to see in the Darkness.

 

You don’t even need to hack long sword proficiency…Sun Blade says you’re already proficient with it if you’re proficient with short swords, so Dedicated Weapon already works on it :)

duplicate post opps!

can a monk use this as a monk weapon?

or Thundarr's blade

 https://en.wikipedia.org/wiki/Thundarr_the_Barbarian 

it's just Thundarr's blade 

I have a monk character with this. 
Selecting Way of the Kensei solves this issue just by making the Longsword one of your Kensei weapons. 
the Kensei monk with this blade is on their way to becoming a literal Jedi 😂

can it be custom colors too?
Like use a pool and what ever number you land is your color that the blade emits.

Sun Blade Colors