On September 2nd 2020, the LIGO-Virgo collaboration announced the detection of GW190521, a gravitational wave source associated with the merger of two black holes (BHs), 66 and 85 Mo masses, which left behind an intermediate-mass black hole (IMBH) with a mass of 142 Mo. This binary merger is peculiar because its primary mass falls in the so-called upper mass-gap, a region of stellar masses where modern stellar evolution predicts the absence of remnants, and the final remnant represents the first specimen of a "light" IMBH. In this seminar, I will describe a novel channel suitable to explain the properties of GW190521, namely a sequence of three mergers among stellar mass black holes. We discovered serendipitously such a process in a high-resolution N-body model of a dense star cluster. We combine these simulations with an analysis based on numerical relativity fitting formulae and on observed properties of globular, young, and nuclear clusters, to show that if GW190521 originated via such a mechanism its observation gives us insights on the distribution of stellar BH natal spins and on the environment that harboured such a system, most likely a dense and young star cluster.