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 an unwanted magic item 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. This rule uses the same chart as the crafting magic items downtime activity as it appears on p.129 of the DMG.
Disenchanting a Magic Item
A character who engages in this activity is disassembling a magic item, and extracting its magical essences into a useful common material, called residuum.
A character must be qualified to craft a given item, in order to disenchant it.
For each workday (8 hours) of disenchanting, the character makes 50 gp of progress, up to the full "value" of the magic item, defined as for crafting it. On completion the magic item is destroyed, and the character obtains a pile of residuum dust, a glowing blue metallic powder, worth half the value (as defined for crafting) of the magic item it was extracted from. Items which cannot be destroyed through mundane means, such as artifacts, are immune to this process.
Residuum counts as a magic item, and can only be sold or purchased through the selling a magic item downtime activity on p.129-130 of the DMG. Residuum itself cannot be disenchanted. Residuum weighs 1 pound per 1000gp of worth (it is worth twice as much per pound as platinum).
Residuum can be used in the place of spell components, if you lack a component pouch or focus, and can be spent to cover the gold coin value of valuable spell components. Residuum can be spent in place of the monetary expense of copying spells into a spellbook by mixing it with ink. Residuum can be used in place of an equal value of silver for the purposes of making holy water or silvered weapons.
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. For example, if you have 50 gp of residuum, and make a magic item with a value of 90 gp, then you only need to craft up to 40 gp of that to complete the item. Residuum can't be used to reduce the crafting time of a magic item below 10 workdays. Once used, residuum is consumed, releasing its metaphysical form from its temporarily physical state. With DM permission, you can use residuum to replace specific rare components a magic item may require in its construction.