Buildup

A Blast of energy is taken by spells (Eldritch Blast, Scorching Ray, etc.), dealing partial damage to you, and absorbing the energy within the selected weapon (Greatsword, Longsword, Arrow for bows, etc.). You roll 4d6 for melee weapons, 3d8 for ranged weapons and add your strength modifier for melee weapons or your dexterity modifier for ranged weapons. This spell can only be used once, and if you fail to use the magic stored within he weapon, it deals the remaining damage. So if an Eldritch Blast is launched and it is supposed to deal 20 damage total, it stores 15 in your sword, dealing 5 damage to you. If you fail to use the magic in your sword, it deals that 15 damage to you.