  The distribution of white dwarf rotation periods provides a means for
constraining angular momentum evolution during the late stages of stellar
evolution, as well as insight into the physics and remnants of double
degenerate mergers. Although the rotational distribution of low mass white
dwarfs is relatively well constrained via asteroseismology, that of high mass
white dwarfs, which can arise from either intermediate mass stellar evolution
or white dwarf mergers, is not. Photometric variability in white dwarfs due to
rotation of a spotted star is rapidly increasing the sample size of high mass
white dwarfs with measured rotation periods. We present the discovery of 22.4
minute photometric variability in the lightcurve of EGGR 156, a strongly
magnetic, ultramassive white dwarf. We interpret this variability as rapid
rotation, and our data suggest that EGGR 156 is the remnant of a double
degenerate merger. Finally, we calculate the rate of period change in rapidly
rotating, massive, magnetic WDs due to magnetic dipole radiation. In many
cases, including EGGR 156, the period change is not currently detectable over
reasonable timescales, indicating that these WDs could be very precise clocks.
For the most highly magnetic, rapidly rotating massive WDs, such as ZTF
J1901+1450 and RE J0317$-$853, the period change should be detectable and may
help constrain the structure and evolution of these exotic white dwarfs.
