Stellar substructures within tidal debris preserve information about theirprogenitor galaxies' properties, offering insights into hierarchical massassembly. We examine a compact stellar system (CSS) around the nearby spiralgalaxy NGC 7531, including the shell-like tidal debris. Our goals are todetermine the nature of the CSS, reconstruct the accretion history, andunderstand how the large, diffuse shell-like structure formed. We presentphotometric measurements of the shell-like debris and CSS using DESI LegacyImaging Survey (LS) data. We obtained Keck/LRIS spectroscopic data for the CSSto confirm its association with NGC 7531 and to derive its star formationhistory (SFH). Deep ($\sim$27.9 mag/arcsec$^{2}$) amateur telescope imagesenabled complete characterization of the tidal debris structure. We confirm theCSS is associated with NGC 7531. We rename it NGC 7531-UCD1, since its stellarmass ($3.7_{-0.7}^{+1.0}\times 10^6$ $\mathrm{M}_\odot$), half-light radius($R_{h} = 0.13 \pm 0.05$ arcsec) and SFH place it as an ultra-compact dwarfgalaxy (UCD). NGC 7531-UCD1 was likely a nuclear star cluster (NSC) that wastidally stripped into a UCD- this is further supported by the presence of tidaltails. We quantify the shell-like debris' mass as $M_\star\sim 3$--$11\times10^8 M_\odot$, implying a merger mass ratio of ~300:1 to 10:1. Our amateurtelescope images confirm new pieces of debris, previously unclear in the DESILS images. N-body simulations reproduce the tidal features, requiring a nearradial orbit of the progenitor with two pericentric passages. The first passagecoincides with the measured star formation enhancement ~1 Gyr ago. Our findingsagree with predictions about the NSC to UCD formation pathway via tidalstripping, and further confirm the presence of these objects outside of ourMilky Way.