  We propose an anisotropy quantifier of the HI 21-cm signal traditionally used
to clock the astrophysics of the reionization era as a post-reionization dark
energy diagnostic. We find that the anisotropy probe can be measured at SNR
$\sim 10$ in both auto-correlation and in cross-correlation with the
Ly-$\alpha$ forest over a wide $z$ and $k-$range. We propose to use the BAO
signature on the anisotropy signal to measure $( H(z), D_A(z))$. Subsequently,
we put constraints on a dark energy model involving a negative cosmological
constant on top of a quintessence scalar field and find that such a model is
consistent with futuristic observations.
