The Dark Knight personality is unlocked when you hit 75 Corruption and then sticks until you get down to 25 or less, though it can be overwritten by the Bimbo personality. The Dark Knight title is unlocked at 100 Corruption
As for how to get that high, early-game events have caps on how high you can go which prevent you from repeating them endlessly. Later one-off and repeatable interactions can get you to 100.
You cannot get 100 Corruption until the events of the Winter City so here's a quick guide in order to get there.
-Spam The River Witch encounter until it caps
-Have sex with Kasyrra every chance you can get
-Drink the cum from Eltharas Ass (Funny enough a one time scene gated behind the corruption softcap which most people wouldn't have at this point in the game)
-Corrupt Jaelyn
-Spam Demon Jaelyn sex in the Wayfort sex until 100 corruption
-Profit