With the recent update to Fallout 76, players can now use a special new item called legendary cores to craft what are called legendary mods. Legendary mods are another new item that can be used to turn non-legendary weapons into legendary ones and reroll existing effects on legendary weapons.
That said, if you want to use legendary cores to make legendary mods, you will first need to get both legendary cores and modules. You can get both of these items from Purveyor Murmrgh in the Rusty Pick located in the Ash heap. You will also need to buy both of these items with scrip, which can be obtained via daily quests, ops, and exchanging legendary weapons via exchange machines.
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These are special machines usually located around stations that can be used to change unwanted legendary weapons for scrip. Once you have scrip through either of these methods, you can then buy both modules and cores from Purveyor Murmrgh or earn them through daily quests, ops, and events.
After that, you can then take both materials to a crafting station and craft them into legendary mods that can then be used to turn items into legendary ones, reroll existing stats, and upgrade legendary items into higher tiers via the modify section of a crafting station.
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