Elemental composition is an essential factor in the chemistry of planetary and brown dwarf atmospheres; however, the majority of low-temperature objects near the Sun have nearly indistinguishable abundance patterns. In this talk, I will review some of the key findings of the JWST Cycle 3 "Arcana of the Ancients" program, which obtained NIRSpec and MIRI spectra for a chemically diverse population of L- and T-type dwarfs (Teff ≈ 500-2500 K), including companions to nearby stars. I report the first detection of undepleted phosphine (PH3) in an atmosphere outside the Solar System, report preliminary analysis of atmosphere elemental abundance patterns between brown dwarfs and their host stars, and a compare the current suite of forward atmosphere models spanning a broad range of temperatures and metallicities, results relevant for interpreting the spectra directly imaged and transiting hot Jovian planets.