  We present new limits on exotic keV-scale physics based on 478 kg d of
MAJORANA DEMONSTRATOR commissioning data. Constraints at the 90% confidence
level are derived on bosonic dark matter (DM) and solar axion couplings, Pauli
exclusion principle violating (PEPV) decay, and electron decay using
monoenergetic peak signal-limits above our background. Our most stringent DM
constraints are set for 11.8 keV mass particles, limiting $g_{Ae} <4.5\times
10^{-13}$ for pseudoscalars and $\frac{\alpha'}{\alpha} < 9.7\times 10^{-28}$
for vectors. We also report a 14.4 keV solar axion coupling limit of
$g_{AN}^{\mathrm{eff}}\times g_{Ae}~<~3.8 \times 10^{-17}$, a
$\frac{1}{2}\beta^2~<~8.5\times10^{-48}$ limit on the strength of PEPV electron
transitions, and a lower limit on the electron lifetime of $\tau_e > 1.2 \times
10^{24}\;$yr for $e^- \rightarrow$ invisible.
