Death Knight's Prowess

You have developed traits similar to those possessed by a death knight, granting you the following benefits:

• Increase your Strength, Constitution, or Charisma by 1, to a maximum of 20.
• While wearing heavy armor, you have immunity to necrotic damage.
• While wearing heavy armor, when you are forced to make a saving throw against a spell or other magical effect, you can use your reaction to give yourself advantage on the saving throw. You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.
• As an action, you can hurl a magical ball of fire that explodes at a point you can see within 120 feet of you. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw (DC 8 + your proficiency bonus + your Charisma modifier). The sphere spreads around corners. A creature takes 5d6 fire damage and 5d6 necrotic damage on a failed save, or half as much damage on a successful one. Once you use this feature, you can't use it again until finish long rest.