The Lesson Taught by Self-Punishment (Snarry)

Having a dormant creature gene ended up being the key to surviving Nagini's bite. The venom mutated his cells and his creature was awoken. The only trade-off; he had to mate soon if he wanted to survive. Now, while there might not be destined mates, the creature does choose another based on their strength and eligibility. When Harry came past the hospital to make sure Snape was okay, the creature flared up and chose him. Severus knows he can't be with Harry. But he will die otherwise. He says nothing and begins to deteriorate. Concerned, Harry demands to know what the problem is so Snape begrudgingly tells him. Harry is torn between accepting and not wanting to at all, but gives in, not wanting Snape to die after everything he'd done. Creature goal completed, Snape turns his back on everything and hides away. When news of Harry Potter's child hits the front page three years later, Severus' creature nearly dies, taking Snape with him. Only Harry can save him. But does he deserve it?
(Obv Harry Potter and co belong to JK Rowling. I created the Elf though, and the child.)