Here's how it happens:
* Basilisk's weakness: The Basilisk is a giant serpent that can kill with a single glance. Its only weakness is the fangs of a phoenix.
* Neville's sword: Harry is disarmed and nearly killed by the Basilisk, but Neville arrives with Gryffindor's sword, which had been embedded in the Sorting Hat. The sword is made of phoenix feather.
* The final blow: Neville uses the sword to kill the Basilisk, saving Harry's life.
Let me know if you have any other questions about the Harry Potter series!