Cleave the Weave

You have mastered runic techniques that sever hostile magic at the moment it strikes. When a spell or magical effect would reduce you to 0 hit points and you already have half your hit points or fewer, you may use your reaction to sacrifice hit points equal to 25% of your maximum to nullify the effect; you take no damage, and one creature within 10 feet you choose also takes no damage. You may use this feature a number of times equal to your proficiency bonus, regaining all uses after a long rest.