The gravitational wave event GW190521 involves the merger of two black holes of $sim 85text{M}_odot$ and $sim 66text{M}_odot$ forming an intermediate-mass black hole (IMBH) of mass $sim 142text{M}_odot$. Both progenitors are challenging to explain within standard stellar evolution as they belong to the upper black-hole mass gap. We propose a dynamical formation pathway for this IMBH based on multiple hierarchical mergers of progenitors in the core of a dense star cluster. We identified such scenarios from analysis of a set of 58 direct N-body simulations using NBODY6-gpu. In one of our canonical runs aimed at describing the evolution of a star cluster with $N=10^5$ stars and typical globular cluster properties, we observe a stellar black hole undergoing a chain of seven binary mergers in 6 Gyr, attaining a final mass of $97.8text{M}_odot$. We discuss the dynamical interactions that lead to the final IMBH product, as well as the evolution of the black hole population in that simulation. From the analysis of all simulations in our dataset we observe additional smaller chains, tentatively inferring that an IMBH formation through chain mergers is expected in the lifetime of a typical (i.e. median mass) globular cluster with probability $0.01 lesssim p lesssim 0.1$. Using this order-of-magnitude estimate and comoving star formation rates we show our results are broadly consistent with the mean rate implied by GW190521, and we discuss implications for future gravitational wave detections of IMBHs.
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