Prismatic Defender
legendary, requires attunement

Most defenders focus on a more straightforward tradeoff between offense and defense capabilities, but the prismatic blade takes a more versatile approach. This +1 greatsword grants damage resistance to 7 damage types, one for every color of the rainbow: Fire (Red), Acid (Orange), Lightning (Yellow), Poison (Green), Cold (Blue), Force (Indigo), and Psychic (Purple). Additionally, damage that is resisted in this way is stored within the blade indefinitely until discharged. So for example, if the wielder would normally take 26 fire damage from a fireball, they instead take 13, and also 13 damage is stored within the blade.

However, the sword can only hold so much, depending on the power of the wielder. For each damage type, it can store up to 10 points times your proficiency bonus. While the sword is "maxed out" for a damage type, it doesn't grant damage resistance against that damage type. If an attack would charge the sword more than it has room for, the wielder will have damage resistance against that whole attack, but the excess charge that would normally grant is lost instead. For example, if a Paladin 14 wielding this blade was hit by the fireball mentioned earlier, but already had 45 fire stored within the blade, they would still take only 13 damage, but only 5 would get stored within the blade.

When the wielder hits using this weapon, they can choose to discharge stored damage as part of the attack. This discharged damage can by any mix of types for which the sword has stored points, but no single discharge can spend more than 5 points times your proficiency bonus. For every 5 points sacrificed of a particular damage type, the hit will deal an additional 1d6 of that damage type. For example, if the Paladin from before hits, they can choose to discharge 25 points; if this is 5 fire, 10 acid, and 10 psychic, then their attack will deal +1d6 fire/+2d6 acid/+2d6 psychic damage.