Magic only starts to return to the world of A Song of Ice and Fire & Game of Thrones after Daenerys' dragons are born, but Valyrian steel weapons remained as potent as ever throughout the centuries when the arcane arts were dwindling in power. Valyrian steel weapons are some of the most prized items in Essos and Westeros, and it's possible to add them to the multiverse.
Valyrian steel was originally forged in ancient times and the secret to its creation has long since been lost, though the greatest blacksmiths in Qohor can reforge the metal without losing its qualities. It's said that Valyrian steel was created using a mixture of dragon fire and arcane arts, but no one knows for sure. A few hundred Valyrian steel weapons exist in the world and many of them belong to the oldest noble families in Westeros. Possession of a Valyrian steel weapon is seen as a status symbol, as only a limited number of them still exist in the world. They are also prized for their incredible durability and sharpness, which is unmatched by anything in the world (with the possible exception of the sword of House Dayne, which was forged from a meteo🌺rite.)
Qualities of Valyrian Steel
A weapon forged of Valyrian steel in Dungeons & Dragons would retain the qualities listed in the Player's Handbook, but they would also gain a +4 to hit and damage. Valyrian steel weapons are said to be able to cut through other metals as if they w🧜ere cloth, so the high bonus is fitting.
It's possible to reforge Valyrian steel into a different weapon, but usually at a high cost, as only the greatest smiths in the world are able to do it. In a standard Dungeons & Dragons world, that means only the greatest living smiths among the elves, dwarves, or gnomes would be ab🌱le to accomplish such a feat,🏅 and they would not do so for cheap.
In A Song of Ice and Fire & Game of Thrones, Valyrian steel can also destroy white walkers when it touches their icy flesh. If a white walker equivalent exists in the D&D ca෴mpaign, then a Valyrian steel weapon should instantly kill them upon a successful hit.
No Magic Weakness
The most important factor regarding the value of Valyrian steel weapons is the fact that they aren't magical. Valyrian steel maintained its strength in the centuries following the decline of magic in their home setting, so it stands to reason that they aren't affected by an absence of magic. As such, Valyrian steel weapons are immune to spells like antimagic field and dispel magic, which means that their bonuses cannot be suppressed by outside effects. This means that their wielder doesn't have to worry about their weapon being made mundane in the mid🐲dle ♑of battle.
A Question Of Value
Valyrian steel weapons are considered to be priceless in Westeros. Tywin Lannister was unable to buy a Valyrian steel weapon to replace the Lannister sword (Brightroar) which had been lost at sea years earlier, as even the poorest noble houses were unwilling to sell a part of their history. If one of the richest men in the world cannot afford a Valyrian steel sword, then it stan꧅ds to reason that it would be beyond the coin purse of all but the wealthiest of D&D heroes.
In a Dungeons & Dragons world, this historical value might not be as present. There are plenty of magic items with powers that equal the effects of Valyrian steel. The secret of its forging would also be difficult to lose, thanks to a number of long-lived races that exist, and magical spells that can uncover how items are created. In a Dungeons & Dragons world, Valyrian steel items would likely be seen as powerful weapons that are useful against spellcasters, but they will lack the vꦕalue they 🐽have in Westeros.
It's possible that the DM could tie them into the history of the setting, as there are many ruined civilizations that could have created these weapons in the past (like Netheril), or the metal needed to create them only comes from a dangerous location (like Chult). There are lots of opportunities to give Valyrian steel weapons the same importance in a world like Faerun or Oerth that they have in Essos and Westeros, so long as the 🌃DM wants to give them a history of their own.