Both a long time ago were the brothers of the protagonist's father/grandparent (Jonathan Joestar and King Randor). Both were unbelievably jealous of their brother and were corrupted by this jealousy and overall their darker more vindictive disposition, both meddled and tampered with things they did not truly understand resulting in their permanent physical change both becoming horrific undead monsters due to this tampering but instead of feeling angered by this cruel twist of fate they embrace it truly becoming the monsters they were destined to become (Dio becoming a vampire via the stone mask and Skeletor becoming a skeleton faced warlock by messing with dark magic). They then tried to fight against their brother but failed in some manner and then proceeded to swear revenge not only against their brother but in particular their brother's bloodline resulting in them holding a grudge and building their power up for decades in order to eventually come back at their brother or more particularly their brother's family. Eventually when they do come back they go head to head with one of their brothers descendants, a young man of imposing stature and physique and inherited mystical powers which allow them to not only project/switch into an alternate persona of even greater power but also the power needed to not just overpower but eventually defeat our bad guy (Jotaro and his stand abilities star Platinum and Prince Adam and the power sword allowing him to transform into He-Man). However this fight for this young protagonist is not easy as our antagonist has an army of super powered individuals who have dedicated their lives to these dark Lords including a right hand woman who is an evil mystic, a left hand man who's a horrific monster and a cavalcade of other wacky strange and surprisingly powerful and sometimes terrifying characters, (Enya the Hag and evil Lyn and Vanilla Ice and Beast Man). Both also possess in some way a dark reflection of the hero's power (Dio with the world and Skeletor with the dark half of the power sword) which makes our heroes struggle all the more harrowing against these indomitable powerhouses of arcane and occult things and truly diabolically evil persuasion. Eventually they are defeated but not only do the echoes of their villainous acts continue to reverberate for eternities against our hero and his family but in some form they both come back and when they do they are far beyond anything that are protagonists have dealt with before ascending to a state of near godhood, above everyone who comes against them (Dio over heaven and Skeletor unleashed, among a few other super forms of his). And with this power they enact a brutal retribution forcing our Heroes into situations where they must team up with other heroes from their own history and even beyond it in order to defeat This truly unstoppable new form of their old adversary for hopefully the final time. Also both are 80s supervillains from franchises well known for their handsome muscle men, pretty women, weird and often nonsensical characters, surprisingly deep lore and poses lots and lots of poses. One is from a sci-fi fantasy franchise the other is from a modern-day fantasy franchise, one manages to kill their brother the other doesn't, one's young relative adversary is a constantly pissed off manly man while the other is an ever jovial smiley guy, one is traditionally shown with the color palette of warmer colors while the other is traditionally shown with the color palette of colder colors.
It's my guilty pleasure MU of sorts