Cocoon of Contingency

You weave a protective glyph of dormant energy into your own vitality. If you cast this spell as a Bonus Action, it consumes a spell slot, triggers immediately, and heals you for 2d8 + your spellcasting modifier.

If cast as a Ritual, the glyph remains dormant for up to 8 hours. If you are reduced to 0 hit points while the glyph is active, it triggers automatically as a magical reflex, immediately enveloping you in a protective cocoon woven from your own hair or raw mystical fibers.

While inside the cocoon, you are Prone and Unconscious, but the cocoon shields you. The cocoon has 10 Hit Points. Any damage you would take is subtracted from the cocoon's hit points instead.

At the start of your next turn, the outcome depends on the cocoon's status:

If the Cocoon survives: It unravels safely. You regain 1d8 hit points, waking up immediately (though you remain Prone on this turn).

If the Cocoon is destroyed: The magical lattice snaps violently. You regain no hit points and remain at 0 hit points (making death saving throws normally), but the backlash opens a breach in the attacker's defenses. One ally within range can immediately use their reaction to make an Opportunity Attack against the creature that destroyed the cocoon.