  We present the first detailed analysis of the ultra-steep spectrum radio halo
in the merging galaxy cluster Abell 521, based on upgraded Giant Metrewave
Radio telescope (uGMRT) observations. The combination of radio observations
(300-850 MHz) and archival X-ray data provide a new window into the complex
physics occurring in this system. When compared to all previous analyses, our
sensitive radio images detected the centrally located radio halo emission to a
greater extent of $\sim$ 1.3 Mpc. A faint extension of the southeastern radio
relic has been discovered. We detected another relic, recently discovered by
MeerKAT, and coincident with a possible shock front in the X-rays, at the
northwest position of the center. We find that the integrated spectrum of the
radio halo is well-fitted with a spectral index of $-1.86 \pm 0.12$. A
spatially resolved spectral index map revealed the spectral index fluctuations,
as well as an outward radial steepening of the average spectral index. The
radio and X-ray surface brightness are well correlated for the entire and
different sub-parts of the halo, with sub-linear correlation slopes
(0.50$-$0.65). We also found a mild anti-correlation between the spectral index
and X-ray surface brightness. Newly detected extensions of the SE relic and the
counter relic are consistent with the merger in the plane of the sky.
