From Destinypedia, the Destiny wiki
An Enhancement Core is an item used to upgrade Legendary gear into Masterworks, as well as for infusion. Prior to Update 2.0.5, they were known as Masterwork Cores. They are obtained by dismantling weapons with the Masterwork rarity, by completing Scrapper bounties, Spider Wanted bounties, or by purchasing them from The Spider for Legendary shards; however, buying them from him doubles the cost during that day. They are also randomly dropped from killing bosses when a Finest Matterweave has been consumed. It takes 10 Cores together with 20 Legendary Shards to Masterwork a Year 1 weapon. Additionally, 3 cores can be used together with 10 Shards to reroll the stats of an already Masterworked Year 1 weapon.
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