Complex sentence: His only known relic is the silver sword Harry used to kill the basilisk, both of which were kept in the office of the headmaster until Severus Snape moved the sword to aid Harry as he destroyed the Horcruxes.
Split into simpler sentences: His only known relic is the silver sword Harry used to kill the basilisk, which was kept in the office of the headmaster. Later, Severus Snape moved the sword to aid Harry as he destroyed the Horcruxes.