Permanently destroys a weapon and adds it to your stock of rift entity weapons. The better you are at this spell, the longer the weapon can be used for before being depleted. The higher the weapon’s power, the longer it can be used before being depleted. Wise riftwalkers complete the ‘clever hands’ task and have their entity ‘heft’ a candidate weapon before using this spell on a valuable weapon.

Unusable or unwanted weapons can be put up for sale in the shaman’s cave.

You could in theory ask a higher end riftwalker to cast the spell and then use the riftwalker shop to sell the weapon to you, although this has obvious risks.

The in game help reads:

No saving throw.
Casting time: 20 rounds.
Type of spell: neutral spell.
Affecting stats: int/wis.
Cast type: rift.
Spell point cost: 261.
This spell is used to add a weapon to the riftwalkers weapon store ('gem
weapons' to list your available weapons). The weapon needs to be in your
inventory, and not in your keep list. Once your weapon is consumed, it will
remain in your weapon store for as long as it has energy. Unfortunately while
your entities make use of your arsenal, the entity will drain the weapons
power. Hence the more powerful the weapon you use, the longer it will last.
Spell chance effects the weapons maximum energy value.
You will be prompted [Y/N] before the spell completely consumes the weapon.
WARNING: The weapon will be PERMANENTLY DESTROYED!
Syntax: cast consume weapon at <weapon/shield>