Many of the weapons you unlock in Elden Ring are “unique” weapons because they have a unique attack art, come from a unique source, or were created from a unique memory. These unique weapons cannot be modified with new Ashes of War types, and also require a different upgrade material than the Forgestones you may have picked up from time to time. To get the most out of your unique weapons you need a very rare resource – so this is where you can find Somber Ancient Dragon Smithing Stones in Elden Ring.
Note: Spoilers for Elden Ring late game progression.
The earliest Sinister Ur-Dragon Forgestone that most players will find is in the crumbling Farum Azula, a late-game zone after you’ve cleared the Giants’ Mountaintop. This zone is very deadly, with a bunch of enemies with high health bars, as well as very deadly eagles that can hit most players with two or three shots. The Somber Ancient Dragon Smithing Stone is in a pavilion just past the Gauntlet of Eagles and Red Lightning Dragons – after the Dragon Temple Rooftop Site of Grace.
If that’s not enough, most players will find the next stone in Leyndall, Ashen Capital. This can only be found after defeating Maliketh the Ebon Blade and setting the Earth Tree on fire. Leyndall is ruined and players must navigate the rubble to reach the throne. A relative of the Ebon Blade will patrol the corpse containing the next Sinister Ancient Dragon Forgestone. You can grab it and run, or you can choose to stay and fight.
https://www.gamepur.com/guides/where-to-get-the-somber-ancient-dragon-smithing-stone-in-elden-ring Where to get the Sinister Ur-Dragon’s Forgestone in Elden Ring