Schematic of the formation model for the Kepler-221 planet system investigated in this study. Sequential migration traps five planets in a chain of first-order resonances (panels a+b). After disk dispersal, the resonance chain breaks (panel c) triggering a dynamical instability that results in the merger of planets d₁ and d₂ (panel d). Convergent migration between planets b and c re-establishes the b, c, and e resonance chain (panel e). On evolutionary timescales (∼1 Gyr) the b, c, and e resonance chain expands to the present-day period ratios due to tidal dissipation (panel f).