The Great Vigor Reap

Eschewing even death, you drain the life-force from your compatriots to turn the tide of impossible battle, gaining the following benefits:

• When reduced to zero HP by a creature at least two size categories larger than you, you may instead regain full HP plus 100 temporary HP in exchange for all your party members within a mile immediately dropping to zero HP.  Party members commence death saves with one automatic death save failure.
• When you gain HP in this way, you immediately gain all the benefits of a long rest.
• Your size and all your belongings grow to match the size of the creature that triggered this effect.
• Spells must be cast at their highest levels while in this form, spell slots permitting.
• Your STR score increases to 25.
• Damage dice from your weapons triple.
• If the triggering creature is reduced to zero HP, the effect ends and you return to your normal size, to unconsciousness, and to zero HP; all other party members reviving at 1 HP each and suffering one level of exhaustion.
• Any party members within one mile who die while this effect is active cannot be resurrected except by means of a Wish Spell.
• If you are reduced to zero HP while this effect is active, you and your party members within one mile die and cannot be resurrected except by means of a Wish Spell.
• If you have used this effect at any time during the game, your alignment can never change.
• If you have used this effect at any time during the game, you can never again be stabilized from zero HP by any means apart from a Wish Spell, 3 successful death saves, or temporarily by using this effect again.
• You regain the use of this effect after one week.