  The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare
observed with IRIS in the near and far ultraviolet. One of the flare ribbons
crossed a sunspot umbra, producing a dramatic, $\sim1000$% increase in the
near-ultraviolet (NUV) continuum radiation. We comprehensively analyze the
ultraviolet spectral data of the umbral flare brightenings, which provide new
challenges for radiative-hydrodynamic modeling of the chromospheric velocity
field and the white-light continuum radiation. The emission line profiles in
the umbral flare brightenings exhibit redshifts and profile asymmetries, but
these are significantly smaller than in another, well-studied X-class solar
flare. We present a ratio of the NUV continuum intensity to the Fe II 2814.45
Ang intensity. This continuum-to-line ratio is a new spectral diagnostic of
significant heating at high column mass (log $m/$[g cm$^{-2}] >-2$) during
solar flares because the continuum and emission line radiation originate from
relatively similar temperatures but moderately different optical depths. The
full spectral readout of these IRIS data also allow for a comprehensive survey
of the flaring NUV landscape: in addition to many lines of Fe II and Cr II, we
identify a new solar flare emission line, He I $\lambda2829.91$ (as previously
identified in laboratory and early-type stellar spectra). The Fermi/GBM hard
X-ray data provide inputs to radiative-hydrodynamic models (which will be
presented in Paper II) in order to better understand the large
continuum-to-line ratios, the origin of the white-light continuum radiation,
and the role of electron beam heating in the low atmosphere.
