  It has been shown that a first order confinement phase transition can
drastically change the relic dark matter abundance in confining dark sectors
with only heavy dark quarks. We study the phenomenology of one such model with
a Z' portal to Standard Model. We find that dark glueballs are long-lived in
this setup and dilute the dark matter abundance after their decay to Standard
Model. With this effect, the correct relic abundance is obtained with dark
matter masses up to $\mathcal{O}(10^6)$~TeV. We find that while a part of the
parameter space is already ruled out by direct detection and collider searches,
there is still a broad space of viable scenarios that can be probed by future
experiments.
