A unit that contains one or more models with an Oath Stone can never choose to flee as a charge reaction, cannot be disrupted and, if any of its models are armed with a hand weapon and a shield, they can make Parry saves against attacks made to their flanks and rear. Furthermore, a character with an Oath Stone must always accept a challenge (if your unit contains more than one character with an Oath Stone, you can choose which of them accepts the challenge).