  We study the cosmological evolution of a light QCD axion and identify the
parameter space to obtain the correct relic dark matter abundance. The axion
potential is flattened at the origin, corresponding to the only minimum, while
it is unsuppressed at $\pi$. These potential features arise by assuming a
mirror sector with the strong CP phase $\bar\theta$ shifted by $\pi$ compared
to the SM sector, which allows the mirror axion potential to be tuned against
the usual QCD axion potential. Before the QCD phase transition, assuming the
mirror sector is decoupled and much colder than the SM thermal bath, the mirror
sector potential dominates, causing the axion to initially roll to a temporary
minimum at $\pi$. However, after the QCD phase transition, the potential
minimum changes, and the axion relaxes from the newly created "hilltop" near
$\pi$ to the CP-conserving minimum at the origin. As the axion adiabatically
tracks this shift in the potential minimum through the QCD phase transition,
with non-adiabatic evolution near $\pi$ and 0, it alters the usual prediction
of the dark matter abundance. Consequently, this "hilltop" misalignment
mechanism opens new regions of axion parameter space, with the correct relic
abundance while still solving the strong CP problem, that could be explored in
future experiments.
