How does Dark magic/Damage work its way in?

[Evir]

One wiki talks about hexes being gone which I've seen so far being true, but replaced now with Dark Pyro/Sorc/Miracles.

How do those work? I can't find any further references about these or changes or how to plan for them. Do all skills have a dark counterpart, like imbue a skill with a dark gem sort of thing?

(fairly new to DS franchise didn't play far into DS1 or DS2)

Trying to theory craft a plan for build first time around, some sort of medium armored fast 1 handed hybrid, with dark magic/shield in offhand.

Lightning spear or something like that? No idea honestly whats possible

[Faceless Ray]

Dark damage that'll be dealt by weapons is listed in the stats of an item, spells don't have a dark counterpart, at least most of them, I saw some guy that invaded me used a sword that on the second strong attack turned into a giant dark sword, so far I've managed to found only 2 weapons with dark magic, it's a Dark Hand (you can buy it from a certain NPC, but I wont spoil you which and how to find it) and some axe that I looted at Lothric wall (sorry, don't know it's name on english because I'm from other country and playing on anoter language, but it has violet mark on it, that means it's been enchanted to darkness), and Darks Hand have an ability that damages with darkness, so I quess most of the dark spells'll come as and ability to some weapons, if you have more questions feel free to ask, but I'm new at ds3 and only at the undead settlement, so I may not have all the answers you needm but I completed ds2 as a dark mage 2 times, though in ds3 a lot of differences in dark magic, for example in ds2 you need to upgrade both int and faith to upgrade darkness and fire damage, but now darkness upgrades from int and faith separately, so good luck to you, hope this information helps and sorry for my bad english.