  We report comprehensive multi-wavelength observations of a peculiar Type
Ia-like supernova ("SN Ia-pec") ASASSN-15pz. ASASSN-15pz is a spectroscopic
"twin" of SN 2009dc, a so-called "Super-Chandrasekhar-mass" SN, throughout its
evolution, but it has a peak luminosity M_B,peak = -19.69 +/- 0.12 mag that is
\approx 0.6 mag dimmer and comparable to the SN 1991T sub-class of SNe Ia at
the luminous end of the normal width-luminosity relation. The synthesized Ni56
mass of M_Ni56 = 1.13 +/- 0.14 M_sun is also substantially less than that found
for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have
generally suffered from large and uncertain amounts of host-galaxy extinction,
which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we
estimate a host extinction for SN 2009dc of E(B-V)_host=0.12 mag and confirm
its high luminosity (M_B, peak[2009dc] \approx -20.3 mag). The 2009dc-like SN
population, which represents ~1% of SNe Ia, exhibits a range of peak
luminosities, and do not fit onto the tight width-luminosity relation. Their
optical light curves also show significant diversity of late-time (>~ 50 days)
decline rates. The nebular-phase spectra provide powerful diagnostics to
identify the 2009dc-like events as a distinct class of SNe Ia. We suggest
referring to these sources using the phenomenology-based "2009dc-like SN
Ia-pec" instead of "Super-Chandrasekhar SN Ia," which is based on an uncertain
theoretical interpretation.
