Disenchanting a Magic Item (5e Variant Rule)
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This downtime activity is inspired by 4th edition and The Elder Scrolls, and pertains to destroying a magic item to extract some abstract resource which can be used to make new magic items. This can be used by a character to convert a useless magic item, (IE: Goggles of night vision owned by an elf) into a partial return that in turn can be used to create a new, more useful item, for cheaper. It can also be used to recycle old magic items into newer, more powerful or more useful versions. That way you aren't lugging around your +1 sword, your +2 sword, your +3 sword, etc. (You know, if your DM is a lazy tool and awarded your magic items that way) This rule uses the same chart as the crafting magic items downtime activity as it appears on p.129 of the DMG. So, to use this, you'll need to crack that book, because I don't feel like reproducing it.
Disenchanting a Magic Item
A character who takes this activity is disassembling a magic item, and extracting its magical essences into a useful common material, called residuum. A character must be at least a high enough level to craft a given item, in order to disenchant it. For each day of disenchanting, the character makes 50gp of progress, up to the full value of the magic item, toward destroying it. Upon destruction of the magic item, they obtain a pile of residuum dust, a glowing blue metallic powder, worth half the value of the magic item it was extracted from. Residuum counts as a magic item, and can only be sold through the selling a magic item downtime activity on p.129-130 of the DMG, but cannot be broken down any farther. Residuum can be used in the place of spell components, if you lack a components pouch or focus, and can be spent to cover the gp value of valuable spell components. Residuum can be spent in the place of the monetary expense of copying spells into a spellbook. Residuum's main purpose is to be spent to reduce the crafting time of a new magic item, by artificially reducing its value for crafting purposes at a 1:1 ratio. (IE, if you have 50gp of residuum, and make a magic item with a value of 90gp, then you only need to craft up to 40gp of that to complete the item. The item is still worth 90gp, you just took a shortcut.) Residuum can not be used to reduce the crafting time of a magic item below 10 days; you still need to spend some real time in using the stuff. Once used, residuum is consumed, releasing its metaphysical form from its temporarily physical state.