Traitor's Shield

Armor (shield), rare (requires attunement)

This shield once belonged to a traitorous knight who was burned for his treason. While holding the shield, you have resistance to necrotic damage.

Damned Rebuke. When you take damage from a creature that you can see within 60 feet of you and are holding the shield, you can use your reaction to cast the hellish rebuke spell from the shield (2nd-level version; save DC 15). You choose whether the spell's damage type is fire or necrotic each time you cast it. Once this property has been used, it can't be used again until the next dawn or until you take 10 or more damage from a single friendly source while holding the shield.

Curse. This shield is cursed, and attuning to it extends the curse to you. While you remain cursed, you are unwilling to part with the shield, and you have disadvantage on saving throws against spells and effects created by friendly creatures.

When you've sold out your friends, all that's left are enemies.