I have this weapon and it helped me and my friend s kill a adult red dragon.Dope.

Quote from Rotheer >>

Yes, a critical hit allows you to roll twice as many dice, no matter the source. A critical hit with a longsword would be 2d8+6d6+1+Str and a greatsword would be 4d6+6d6+1+Str. Bilbo the 5th-level rogue with 3d6 sneak attack and a rapier would do 2d8+6d6+6d6+1+Dex!

 What about superiority dice?

TO OVERPOWERED seriously.

Quote from BerzerkRaijin >>

Quote from Rotheer >>

Yes, a critical hit allows you to roll twice as many dice, no matter the source. A critical hit with a longsword would be 2d8+6d6+1+Str and a greatsword would be 4d6+6d6+1+Str. Bilbo the 5th-level rogue with 3d6 sneak attack and a rapier would do 2d8+6d6+6d6+1+Dex!

 What about superiority dice?

 Yes, superiority dice are doubled just the same.

Basically any dice (not modifiers) that you would roll without a critical hit are rolled twice

If a critical hit is the prerequisite (i.e. vorpal swords dealing an extra 6d10), it’s not doubled, since it wouldn’t exist without the critical.

I use the Selling Magic Items price table as reference, rare items are listed as betweeen 501 and 5.000gp, I just pick a price in that range that feels right for the level of power the item offers. 

Can it be made out of Adamantine

Quote from ClericalKenku >>

Quote from BerzerkRaijin >>

Quote from Rotheer >>

Yes, a critical hit allows you to roll twice as many dice, no matter the source. A critical hit with a longsword would be 2d8+6d6+1+Str and a greatsword would be 4d6+6d6+1+Str. Bilbo the 5th-level rogue with 3d6 sneak attack and a rapier would do 2d8+6d6+6d6+1+Dex!

 What about superiority dice?

 Yes, superiority dice are doubled just the same.

Basically any dice (not modifiers) that you would roll without a critical hit are rolled twice

If a critical hit is the prerequisite (i.e. vorpal swords dealing an extra 6d10), it’s not doubled, since it wouldn’t exist without the critical.

First off, vorpal swords deal 6d8 extra, not 6d10. Second off, the 6d8 bonus damage is not inflicted on a critical hit, it is inflicted on a nat 20. This is an important distinction since anyone wearing adamantine armor (or something else that provides crit immunity) still takes the 6d8. Also, due to that same reason, the 6d8 bonus damage would be doubled to 12d8 if the attack crits. Some features also allow crits between 19 and 20, etc. and a crit on a 19 from that would not trigger the extra 6d8 damage.

Source: https://www.sageadvice.eu/2016/07/30/vorpal-sword-does-the-6d8-get-doubled/?fbclid=IwAR0kcOry_am6UBvZYE0GzXTrLb-BrEjSMpXzh_sFpF1Jq4FKevkPFoecPlE

Well they are related in FR. The IO thing is a religious myth of theirs sure, but IO is also known as Asgorath, the progenitor of all dragons. So if its true, then dragonborn would absolutely be related to dragons. But that doesn't mean this weapon hurts them more. You don't make a weapon like this to kill humanoids, it's for killing the real deal.

In fact, in FR, the dragonborn were slaves to their cousins in Abier. The ones who were left in Toril post-Second Sundering are the only ones who threw off those shackles, and to this day despise all true dragons. So they probably make these weapons as often as they can, and it'd be silly to make a weapon that deals extra damage to yourself.

I found this a bit underwhelming, so if you prefer...

Balthazar The Dread’s Dragon Slayer

You gain a +2 bonus to attack and damage rolls made with this magic weapon.

When you hit a dragon with this weapon, the dragon takes an extra 3d6 damage. For the purpose of this weapon, "dragon" refers to any creature with the dragon type, including dragon turtles and wyverns (let’s go with the MM category for “type”, the internet has some differences of opinion on this matter)

Once per long rest: the wielder can cast Dragon’s Breath spell ON HIM/HERSELF as an action, but otherwise as per the spell description (this is concentration), the type of damage must be of the last dragon slayed by this weapon (in this case: lightning). The Dragon Slayer must be touched for casting and maintaining this spell.

As a bonus action: you can locate the direction of a dragon “type” creature within 150 feet

As a bonus action: you can send a telepathic message to a dragon “type” creature within 150 feet

Saving against a dragon’s frightful presence is made at Advantage.

For Attunement: Spend 1 hour in meditation with the sword convincing it that you would love to kill dragons, and it will transform into the melee/finesse weapon of your choice (not ranged)

Attunement needed.

It’s powerful, sure. But, like... what a boring weapon.

“That thing was too big to be called a sword. Too big, too thick, too heavy, and too rough, it was more like a large hunk of iron.”

R.I.P. Miura Sensei

What rarity?

Is a good price for this around 1500 gp? my sis (who was DMing for me) made a shop with literally everything and I got this plus a green dragon scale mail for 2000gp (that was probably too cheap lol)

Yes? The adamantine rules don't make a weapon magic and just list a flat price to upgrade a weapon. The adamantine weapon rule is odd as it apparently stacks with everything? Talk to your GM I guess.

Let’s just say, more than enough 

Is it the metal or an enchantment that gives it the property of being more effective against dragons?

Quote from PootisMasta101 >>

Is it the metal or an enchantment that gives it the property of being more effective against dragons?

 Enchantment.

Quote from Luqaslima >>

No, Dragonborns are Humanoid. But applies to Faerie Dragons, Dragon Turtles and Kobolds.

 Kobolds are Humanoid, not Dragon (by RAW).

I would say that's subjective to it's demand. It's certainly important and valuable for being good at slaying dragons, but does the Universe or area that the seller in have a lot of dragons. This is a weapon you can build a whole sidequest line for in order to help the players defeat a Dragon in a story. You could make it just a relic item that's free or a gift because they might run into wyverns and it will help there but wont be relevant to greater threats. Id Price it at starting

Dragon Relevant World 800-1200gp
Dragon is the Final Boss 3000-5000gp m(Should really be a quest reward imo)
Dragons are extinct 500-1000gp (Maybe a collectible item and you can throw 1 dragon at the players if they decided to buy so they feel it was relevant)
Dragons are rare but few ( 1500-2500gp) (Should really belong to an old dragonslayer npc or someone associated closely to the weapon and should def be earnable for the party, maybe as a sign of faith or respect

If a bronze dragon turned into a different creature, would the sword still work on it?