  Although true metal-free "Population III" stars have so-far escaped
discovery, their nature, and that of their supernovae, is revealed in the
chemical products left behind in the next generations of stars. Here we report
the detection of an ultra-metal poor star in the Sculptor dwarf spheroidal
galaxy, AS0039. With [Fe/H]$_{\rm LTE}=-4.11$, it is the most metal-poor star
so far discovered in any external galaxy. Contrary to the majority of Milky Way
stars at this metallicity, AS0039 is clearly not enhanced in carbon, with
[C/Fe]$_{\rm LTE}=-0.75$ and A(C)=+3.60, making it the lowest detected carbon
abundance in any star to date. It furthermore lacks $\alpha$-element
uniformity, having extremely low [Mg/Ca]$_{\rm NLTE}=-0.60$ and [Mg/Ti]$_{\rm
NLTE}=-0.86$, in stark contrast with the near solar ratios observed in C-normal
stars within the Milky Way halo. The unique abundance pattern indicates that
AS0039 formed out of material that was predominantly enriched by a $\sim$20$
M_\odot$ progenitor star with an unusually high explosion energy
$E=10\times10^{51}$ erg. The star AS0039 is thus one of the first observational
evidence for zero-metallicity hypernovae and provides a unique opportunity to
investigate the diverse nature of Population III stars.
