  The relic abundance of the dark matter (DM) particle $d$ is studied in a
secluded DM scenario, in which the $d$ number decreasing process dominantly
occurs not through the pair annihilation of $d$ into the standard model
particles, but via the $dd \to mm$ scattering process with a subsequently
decaying mediator particle $m$. It is pointed out that the cosmologically
observed relic abundance of DM can be accomplished even with a massive mediator
having a mass $m_m$ non-negligibly heavy compared with the DM particle mass
$m_d$. In the degenerated $d$-$m$ case ($m_d=m_m$), the DM relic abundance is
realized by adjusting the $dd \to mm$ scattering amplitude large enough and by
choosing an appropriate mediator particle life-time. The DM evolution in the
early universe exhibits characteristic "terrace" behavior, or two-step number
density decreasing behavior, having a "fake" freeze-out at the first step.
Based on these observations, a novel possibility of the DM model buildings is
introduced in which the mediator particle $m$ is unified with the DM particle
$d$ in an approximate dark symmetry multiplet. A pionic DM model is proposed to
illustrate this idea in a renormalizable field theory framework.
