When a creature you can see hits an ally within 5 feet of you with an attack roll, you can take a Reaction to force the attack to target you instead. If the attack roll equals or exceeds your AC, the attack hits you instead; otherwise, it misses.
Once you use this feature, you can't use it again until you finish a Short or Long Rest unless you expend one Battle Die (no action required) to restore your use of it.