Cybersickness remains an obstacle to widespread adoption of XR HMDs, and cybersickness detection and mitigation present the possibility of removing this obstacle. Numerous physiological signals have been previously used to detect cybersickness, but there is no comprehensive single source of truth for developers to consult or build their own cybersickness mitigation off. We integrate existing literature on biometric wearables, electroencephalography, and oculometry to create a Composite Cybersickness Index which provides real-time information to application developers on how cybersick the participant is and what forms of cybersickness they are manifesting. We show this Composite Cybersickness Index is capable of driving real-time mitigation in a VR application. This Index has the potential to break down barriers to access for XR HMDs, and provides the scholarly community with a single, coherent language for assessing and mitigating cybersickness in the future.