Nuclear star clusters (NSCs) are the densest stellar systems in the universe with masses from 10^6-10^7 Msol and half-light radii of about 2-5 pc. They are found in a high fraction of galaxies across the Hubble sequence, including dwarf galaxies. NSCs are known to co-exist with black holes, being the Milky Way the best example. The origin of some of the most massive and metal complex Galactic globular clusters (GCs) is still unclear. One of the proposed explanations is they are former NSCs of dwarf galaxies accreted and disrupted by the Milky Way. In this context, the nucleus of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), M54 (1-2x10^6 Msol), at a distance of 28.4 Kpc offers a privileged view of a nucleus in a stage before the complete stripping of the host galaxy. This complex nucleus provides a link between high-mass metal complex globular clusters and nuclear star clusters. We performed an analysis of ~6600 member stars extracted from MUSE observations of this nucleus. This wealth of chemo-dynamical information allowed the study of a galactic nucleus at an unprecedented detail, unveiling the presence of at least three stellar sub-populations with clear differences in age and metallicity. I will illustrate how these populations can be explained by a combination of events in the nucleus of the Sgr dSph galaxy. Our data show evidences that the build-up of this nucleus could be the result of the interplay between infall and accretion of GCs by dynamical friction effects and of in-situ star formation from gas inflows.