Massive early-type galaxies follow a tight relation between the mass of their central supermassive black hole ( M _ { BH } ) and their stellar mass ( M _ { \star } ) . The origin of observed positive outliers from this relation with extremely high M _ { BH } ( > 10 ^ { 9 } M _ { \odot } ) remains unclear . We present a study of such outliers in the Hydrangea/C-EAGLE cosmological hydrodynamical simulations , designed to enable the study of high-mass galaxy formation and evolution in cluster environments . We find 69 M _ { BH } ( M _ { \star } ) outliers at z = 0 , defined as those with M _ { BH } > 10 ^ { 7 } M _ { \odot } and M _ { BH } / M _ { \star } > 0.01 . This paper focusses on a sample of 5 extreme outliers , that have been selected based on their M _ { BH } and M _ { \star } values , which are comparable to the most recent estimates of observed positive outliers . This sample of 5 outliers , classified as ‘ ‘ Black hole monster galaxies ’ ’ ( BMGs ) , was traced back in time to study their origin and evolution . In agreement with the results of previous simulations for lower-mass M _ { BH } ( M _ { \star } ) outliers , we find that these galaxies became outliers due to a combination of their early formation times and tidal stripping . For BMGs with M _ { BH } > 10 ^ { 9 } M _ { \odot } , major mergers ( with a stellar mass ratio of \mu > 0.25 ) at early times ( z > 2 ) precede the rapid growth of their supermassive BHs . Furthermore , the scatter in the relation between M _ { BH } and stellar velocity dispersion , \sigma , correlates positively with the scatter in [ Mg/Fe ] ( \sigma ) . This indicates that the alpha enhancement of these galaxies , which is closely related to their star formation history , is related to the growth of their central BHs .