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Transformations accompanying shape-instability govern the morphological configuration and distribution of the phases in a microstructure. Owing to the influence of the microstructure on the properties of a material, in the present work, the stability of three-dimensional rods in a 'representative' polycrystalline system is extensively analysed. A multiphase-field model, which recovers the physical laws and sharp-interface relations, and includes grain boundary diffusion, is adopted to investigate the morphological evolution of the precipitate. Moreover, the efficiency of the numerical approach is ensured by establishing the volume-preserving chemical equilibrium through the incorporation TCFe8 (CALPHAD) data and solving phase-field evolution in the Allen-Cahn framework. The morphological evolution of the rod in the representative multiphase system exhibits a unique transformation mechanism which is significantly different from the evolution of an isolated finite-structure. It is realised that, in a polycrystalline arrangement, irrespective of the initial size of the rod, the shape-change begins with the energy-minimising events at the triple junctions. This early transformation renders a characteristic morphology at the longitudinal ends of the structure, which introduces sufficient driving-force through the curvature-difference for the subsequent morphological changes. The continued mass transfer to the terminations, ultimately, breaks-off the rod into separate entities that are entangled in the grain boundary. With increase in the aspect ratio of the rod, it is identified that the source of mass transfer, which turns into the ovulation site, shifts from the centre. This increases the number of fragmentation events and introduces satellite particle. The size of the satellite particle is dictated by a definite ovulation criterion, which is ascertained by examining the transformation of different-sized rods. A comprehensive understanding of the transformation kinetics and mechanism governing the morphological evolution of the rods in a polycrystalline system is rendered in this work. [ABSTRACT FROM AUTHOR]