So, I learned about this series from a CK3 mod called World of Darkness, which changes the game to reflect the political situation of the Vampire World during the Middle Ages.
It's gotten me a bit interested in this. I was wondering if someone can provide me some helpful places to obtain more info on these games and their settings and also possibly tell me how difficult would it be to get into if you're just starting out.
If you're new to these games, then there's one big confusing thing we need to get out of the way: there are two different settings called the World of Darkness.
The original World of Darkness (now typically called the Old World of Darkness, or oWoD) began with Vampire: the Masquerade in the early 90's. While it includes multiple other games, Vampire: the Masquerade, Werewolf: the Apocalypse, and Mage: the Ascension are viewed as the "big three." It was pretty heavy on metaplot, with new supplements and editions detailing new events that happened in the setting, ultimately culminating in the Time of Judgement scenarios that basically blew up the setting for the new millennium. This is the setting that tends to get featured in video games and stuff (and probably the mod you're using as well). New material for oWoD games is still being published (often in the form of anniversary editions of the games), though the target audience is primarily older fans of the franchise rather than new players.
In the early 2000's, the setting was rebooted into the New World of Darkness (later rebranded to Chronicles of Darkness). It's often referred to as nWoD or CofD. It features revamped and reimagined versions of the older games, featuring much of the same general concepts but with varying different themes, mechanics, and settings. The "big three" are Vampire: the Requiem, Werewolf: the Forsaken, and Mage: the Awakening. In general, nWoD sought to ditch the 90's grunge and the metaplot, opting for a more gothic tone and a smaller, more localized scope with the setting instead of the globe spanning conspiracies of oWoD.
I would personally recommend nWoD over oWoD to a newcomer. The games are tighter and more mechanically consistent, the lore is easier to get into without the mountains of metaplot, and, well... some elements of the oWoD didn't age particularly well. I understand there's been something of an effort to clean things up in the anniversary editions, but there's still some stuff in there that's rather uncomfortable to a modern audience.
I hope that helps clear up some of the initial confusion, but if you've got any questions I'm happy to answer them! It's always fun to introduce somebody to something you like, and I really like these games.