  Simplified models of the dark matter (co)annihilation mechanism predict
striking new collider signatures untested by current searches. These models,
which were codified in the coannihilation codex, provide the basis for a dark
matter (DM) discovery program at the Large Hadron Collider (LHC) driven by the
measured DM relic density. In this work, we study an exemplary model featuring
$s$-channel DM coannihilation through a scalar diquark mediator as a
representative case study of scenarios with strongly interacting coannihilation
partners. We discuss the full phenomenology of the model, ranging from low
energy flavor constraints, vacuum stability requirements, and precision Higgs
effects to direct detection and indirect detection prospects. Moreover,
motivated by the relic density calculation, we find significant portions of
parameter space are compatible with current collider constraints and can be
probed by future searches, including a proposed analysis for the novel
signature of a dijet resonance accompanied by missing transverse energy (MET).
Our results show that the $13$ TeV LHC with $100~\mathrm{fb}^{-1}$ luminosity
should be sensitive to mediators as heavy as 1 TeV and dark matter in the
400--500 GeV range. The combination of searches for single and paired dijet
peaks, non-resonant jets + MET excesses, and our novel resonant dijet + MET
signature have strong coverage of the motivated relic density region,
reflecting the tight connections between particles determining the dark matter
abundance and their experimental signatures at the LHC.
