I mean, thinking about it for a Primarch that rarely gets talked about even with modern 40k trying to pimp him out a bit, Mortarion has a lot of insane lore attached to him. Cause ya have this basic concept; hes a guy whos really hard to kill cause hes got insane stamina, hes really into poisons and can resist them to an insane level, hes a social Darwinist who believes in survival of the fittest, hes a close confidant of horus and trusts him enough to follow him into rebellion, and he eventually falls to nurgle and becomes skeleton mothman.
except, also, in the lore around him hes so into poison that he drinks the stuff before and after battle to celebrate as casually as water
and also, its considered an honour to drink poison with him among his legion, poison that will cause the hardiest astartes in the imperium to start puking out their guts
and also, hes a chemical expert on poisons to the point of making his owns
and also, hes an expert biologist
and also, hes an expert mad scientist in general who builds anti warp devices
and also, he believes in numerology
and also, hes such an expert in numerology that he can use it to create scientific devices and mess with warp shit
oh, and also, hes a master class alpha level psyker but he represses it and refuses to use his powers right up until his fall
and also, hes a master engineer and designer who can scrap together the resources and know how to build radiation suit armor on a feral world in a large enough quantity to pimp out an entire army with and also just design and build his own unique machines of war like the plagueburst crawler after his fall
and also, he has like a ton of glorina class battleships on hand that he can just hand them out willy nilly, like he gets one, his most trusted subordinate gets one, he has enough to have some spare ones lurking in his fleet, maybe he has one of the best primarch fleets maybe he doesnt. no one knows nor has cared to explain.
and also he has stockpiles of planet breakers on hand, like an unreasonable number of them, enough of them that he just keeps them in a garage on his new plague planet in 40k and you have to get a permit from his cult of numerologists to rent one
also he has a cult of numerologists
and also, maybe he just wanted to be a farmer instead of this intergalactic general bullshit. like, the first thing he does coming down the POISON DEATH MOUNTAIN TOO DANGEROUS FOR THE HARDIEST PRIMARCH after escaping his EGOMANIACAL AND ABUSIVE NECROMANCER SCIENTIST ALIEN NOT ALIEN ADOPTIVE FATHER WHO IS IMPLIED TO HAVE LEFT MENTAL SCARS AS DEEP AND FUCKED UP AS ANGRONS ON MORTARIONS MIND, is that instead of rallying the human settlements and establishing himself as a ruler like most of the other primarchs at his age he just.. farms. he learns how to farm, cut wheat, feed the space chickens, that kind of stuff.
also, he might not even actually believe in social dawarnisim cause hes noted as having pushed him and his legion to fight the hardiest and most dangerous of enemies to the imperium out of a sense of moral obligation. Like, the space cthullu level shit as opposed to burning civilians like konrad.
also, he was friends with konrad and that has received no explanation and further exploration outside of that fact.
also, half of his face could get exploded off and hed be mostly fine
also, he was a stealth ninja able to sneak through the imperial palace
also, he found out about the golden throne and warp gates existence during his first visit to the imperial palace
also, he might have been an addict or dependent on his planets toxic gas
also, he was apparently one of the strongest primarchs
also, he was apparently one of the best generals of the primarchs
also, the impetus of his fall to chaos was that he couldn’t bare seeing his legion suffer the destroyer hive virus any more then they already were during their impromptu visit to the warp
and like, i could probably go on but ya get my point.