  We develop a new phenomenological model that addresses current tensions
between observations of the early and late Universe. Our scenario features: (i)
a decaying dark energy fluid, which undergoes a transition at $z \sim 5,000$,
to raise today's value of the Hubble parameter -- addressing the $H_0$ tension,
and (ii) an ultra-light axion, which starts oscillating at $z\sim 16,000$, to
suppress the matter power spectrum -- addressing the $S_8$ tension. Our Markov
Chain Monte Carlo analyses show that such a Dark Sector model fits a
combination of early time datasets slightly better than the $\Lambda$CDM model,
while reducing both the $H_0$ and $S_8$ tensions to $\lesssim 3\sigma$ level.
Combined with measurements from cosmic shear surveys, we find that the
discrepancy on $S_8$ is reduced to the $1.4\sigma$ level, and the value of
$H_0$ is further raised. Adding local supernovae measurements, we find that the
$H_0$ and $S_8$ tensions are reduced to the $1.5\sigma$ and $1.1\sigma$ level
respectively, with a significant improvement $\Delta\chi^2\simeq -17$ compared
to the $\Lambda$CDM model. We discuss a possible particle physics realization
of this model, with a dark confining gauge sector and its associated axion,
although embedding the full details within microphysics remains an urgent open
question. Our scenario will be decisively probed with future CMB surveys.
