On the top floor of the Hyrule Castle 1F there is Royal Broadsword, and when you go on top of the roof there is Royal Halberd. On the way there you can get weapons as well so it is recommended.
Collect Other Weapons As Well
Inside the Hyrule Castle, there are Royal Guard, Royal series, Dusk Bow and others, so it has a variety of items. By unlocking the nearby Shrine and using Ascend, you can get several equipments without fighting enemies.
Hyrule Castle Map
Treasure Chest In Bokoblin Base In East Mount Dunsel
After defeating all of the Bokoblins then you can open the treasure chest.
Available From Caves
Goronbi River Cave
Walnot Mountain Cave
Mount Dunsel Cave
Akkala Citadel Ruins Cave
Fort Hateno Cave
Death Mountain Foothill Cave
Hebra South Summit Cave
Crenel Peak Cave
Ulria Grotto East Cave
Tabahl Woods Cave
Where To Get The Pristine Royal Halberd
The Shadow Soldier Which Drops The Royal Halberd
Available From Shadow Soldiers When Decayed Weapons Are Broken
The Pristine Royal Halberd can be obtained from the Shadow Soldier in the Depths. Note that the Undecayed Weapon will not appear unless you destroy the Decayed Weapon of the same name, which can be obtained on the Surface, so when obtaining the weapon, destroy it first.
Available Whenever A Red Moon Occurs
Shadow Soldiers respawn during the Red Moon, allowing you to obtain the Pristine Weapons repeatedly.