Legendary mods are a new material in Fallout 76, and you can get it by crafting it from both legendary modules and cores to upgrade non-legendary weapons/armor into legendary ones.
You can also use these mods to reroll stats and effects for legendary weapons/armor and even upgrade existing legendary items into higher-tier versions.
That said, if you want to get legendary mods, you will need to get both legendary modules and cores, and you can get the modules from the mole miner merchant at the rusty pick for scrip. But for legendary cores, you will need to earn them through daily ops, events, and daily quests.
You can also earn Scrip is through daily ops, events, and daily quests. But other than these activities, there is another way you can earn scrip in the game, and it is by exchanging legendary weapons at exchange machines at train stations.
Moving on, once you got the scrip, you can use it to buy the legendary cores and modules you need to craft legendary mods. You will also need a certain amount depending on the version you want to make, as shown above.
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