Findings indicate that software system modernisation projects are often more complex and costly than initially thought, and that they risk failure as a result. Layered in business processes, workflows, historical implementation and evolution decisions, legacy systems have remained in use because they have been useful and not easily replaced. Modernizing these systems can involve partial functional migration or entire re-implementation, the extent to which is utilized requires deep upfront planning and investigation. In this paper, we provide a multivocal literature review (MLR) of Software System Modernisation, examining its ongoing relevance amidst continuous technological advancements. Our research provides an analysis on the role of a legacy system, the need and benefits of legacy system modernisation, various strategies and approaches employed, challenges encountered throughout the modernisation process and the risks and costs that shape modernisation endeavours. This systematic investigation of the topic of modernisation can highlight the various considerations for researchers and practitioners tasked with modernisation evaluations.