  We present the Dark Energy Survey (DES) discovery of DES15E2mlf, the most
distant superluminous supernova (SLSN) spectroscopically confirmed to date. The
light curves and Gemini spectroscopy of DES15E2mlf indicate that it is a Type I
superluminous supernova (SLSN-I) at z = 1.861 (a lookback time of ~10 Gyr) and
peaking at M_AB = -22.3 +/- 0.1 mag. Given the high redshift, our data probe
the rest-frame ultraviolet (1400-3500 A) properties of the SN, finding velocity
of the C III feature changes by ~5600 km/s over 14 days around maximum light.
We find the host galaxy of DES15E2mlf has a stellar mass of 3.5^+3.6_-2.4 x
10^9 M_sun, which is more massive than the typical SLSN-I host galaxy.
