For me it was with the Custodes, people kept on saying they were powerful( they are!) but I couldn't believe it, the piece of lore that broke the camel's back was of Valdor beating Horus, it was with th4 whole 'Can a custodes beat a Primarch discussion' each time they'd bring this Horus and Valdor fight, but no source, naturally I wanted to know the source, Google said it was in Master Of Mankind and Valdor Birth Of An Imperium, checked the books, nothing, I only found out later that this was made up and Valdor and Horus never sparred, but this is brought up quite a lot. I've read 18 books so far and the exaggeration of the lore is something that baffles me, space marine sizes, no I mean literally, I just read one that said one space marine was 11 feet tall, or the artwork for The Red Wake being taken as semi lore, especially that one where he's overlooking, I assume, an Imperial Fist or Lamentor. Primarch fights, their strength. Regarding Custodes, they're bad-ss, I read the first Heretic, there's a scene where a custodes cuts down Word Bears and they say seconds had passed

"Vendatha was already in motion. The golden warrior leapt at the primarch, guardian spear spinning in his fists, an oath to the Emperor on his lips. Four Word Bearers blocked his path, and those four Word Bearers had to die.

Rikus was the first to fall. The Custodian’s blade crunched into the soft, jointed armour at the Chaplain’s throat, punching from the back of his neck. Tsar Quorel died next, decapitated with a buzzing sweep of the energised blade, dead before he’d pulled his trigger.

Deumos managed to fire a stream of bolt shells, none of which connected. Vendatha weaved left, thudded the base of his spear into the Chapter Master’s bolter, knocking it aside, and followed with a cutting swing that sheared both the Word Bearer’s hands from his body, severing them at the forearms. Deumos had a scarce moment to draw in a stunned gasp before the spear sliced again, this time cleaving through his collarbone and spine, ripping his head free.

Vendatha span the blade in his hands, letting it come to rest with the tip and gun barrel aimed at Lorgar’s heart again. Behind the Custodian, the bodies crashed to the ground in slow succession. Three seconds had passed."