Xenia is a medium-sized mission optimized to study cosmic reionization, cluster formation and evolution, and the WHIM, following cosmo-chemical evolution from the very earliest times to the present. Reconstructing the cosmic history of metals, from the first population of stars to the processes involved in the formation of galaxies and clusters of galaxies, is a key observational challenge. Most baryons reside in diffuse structures, in (proto)-galaxies and clusters of galaxies, and are predicted to trace the vast filamentary structures created by the ubiquitous Dark Matter. X-ray spectroscopy of diffuse matter has the unique capability of simultaneously probing a broad range of elements (C through Fe) in all their ionization stages and all binding states (atomic, molecular, and solid), and thus provides a model-independent survey of the metals. Xenia - proposed to the Astro2010 Decadal Survey - will combine cryogenic imaging spectrometers and wide field X-ray optics with fast repointing to collect essential information from three major tracers of metals: Gamma Ray Bursts (GRBs), Galaxy Clusters, and the Warm-Hot Intergalactic Medium (WHIM). We give an overview of the mission and discuss the instruments designed to carry out these observations.