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Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eyebrightness and has been detected from radio to $\gamma$-rays. An extremely fastoptical decline of 2 magnitudes in 1.2 days and strong Ne lines imply ahigh-mass white dwarf. The optical pre-outburst detection of a 501.42soscillation suggests a magnetic white dwarf. This is the first time that anoscillation of this magnitude has been detected in a classical nova prior tooutburst. We report X-ray outburst observations from {\it Swift} and {\itChandra} which uniquely show: (1) a very strong modulation of super-soft X-raysat a different period from reported optical periods; (2) strong pulse profilevariations and the possible presence of period variations of the order of0.1-0.3s; and (3) rich grating spectra that vary with modulation phase and showP Cygni-type emission lines with two dominant blue-shifted absorptioncomponents at $\sim 3000$ and 9000 km s$^{-1}$ indicating expansion velocitiesup to 11000 km s$^{-1}$. X-ray oscillations most likely arise frominhomogeneous photospheric emission related to the magnetic field. Perioddifferences between reported pre- and post-outburst optical observations, ifnot due to other period drift mechanisms, suggest a large ejected mass for sucha fast nova, in the range $2\times 10^{-5}$-$2\times 10^{-4} M_\odot$. Adifference between the period found in the {\it Chandra} data and a reportedcontemporaneous post-outburst optical period, as well as the presence of perioddrifts, could be due to weakly non-rigid photospheric rotation.

The Remarkable Spin-down and Ultra-fast Outflows of the Highly-Pulsed Supersoft Source of Nova Hercules 2021