Dusk Claymore is a two-handed sword found in Tears of the Kingdom.
One of the handful of special legacy Zelda weapons that can be both found in-game and through scanning amiibo, its design is modelled after the Sword of the Six Sages from The Legend of Zelda: Twilight Princess and uses the model of the amiibo-only sword of the same name from The Legend of Zelda: Breath of the Wild. Unlike the sword in the latter game however, the Dusk Claymore has a compendium entry, listed at #393.
Tears of the Kingdom
The Dusk Claymore is sealed away in a chest in the Thyphlo Ruins, which can be found after completing a Side Adventure from Kazul. Before starting the quest, all four Regional Phenomena must be completed. Kazul is found opposite of the Thyphlo Ruins Skyview Tower. After talking to Kazul, read the monolith, which will give Link each quest.
The Dusk Claymore can also be obtained earlier by scanning the Ganondorf (Super Smash Bros.) amiibo using the amiibo rune. It can subsequently be purchased from any Bargainer Statue found in the Depths after being obtained.
The first quest is called The Owl Protected by Dragons. For this quest, head straight if facing the monoliths. Link will find a large owl with a pedestal around a few dragon heads. Use Tulin's Power of Wind to reveal the first chest, which contains three Sapphires. This completes the first quest.
The second quest is called The Corridor between Two Dragons. This time head west - on the map will be a bridge between a few long dragons. Use Yunobo's Power of Fire and channel it through the passage to reveal a secret chest with three Rubys.
The third quest is called The Six Dragons. Head northwest to find a small lake with six dragon heads. While using Sidon's Power of Water, charge up and release the water power to reveal the next chest with five Opals.
The fourth quest is called The Long Dragon. This time head east to find a very long dragon. Walk over to the tail and shoot an arrow using Riju's Power of Lightning. The next chest will appear with three Topaz.
After completing all of the quests a new area will appear. Head inside and to the right to find the Dusk Claymore. Before leaving, read the secret monolith to get a reward. The Dusk Claymore can subsequently be bought from any Bargainer Statue in the Depths.