  The detonation of a helium shell on a white dwarf has been proposed as a
possible explosion triggering mechanism for Type Ia supernovae. Here, we report
ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent
with being a helium-shell double-detonation. With a rise time of $\approx 18$
days from explosion, the transient reached a peak absolute magnitude of $M_R
\approx -18.2$ mag, exhibiting a light curve akin to sub-luminous SN
1991bg-like Type Ia supernovae, albeit with an unusually steep increase in
brightness within a week from explosion. Spectra taken near peak light exhibit
prominent Si absorption features together with an unusually red color ($g-r
\approx 2$ mag) arising from nearly complete line blanketing of flux blue-wards
of 5000 \AA. This behavior is unlike any previously observed thermonuclear
transient. Nebular phase spectra taken at and after $\approx 30$ days from peak
light reveal evidence of a thermonuclear detonation event dominated by Fe-group
nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are
consistent with the detonation of a massive ($\approx 0.15$ M$_\odot$) helium
shell on a sub-Chandrasekhar mass ($\approx 0.75$ M$_\odot$) white dwarf after
including mixing of $\approx 0.2$ M$_\odot$ of material in the outer ejecta.
These observations provide evidence of a likely rare class of thermonuclear
supernovae arising from detonations of massive helium shells.
