dark souls has one of, if not THE best online implementation in gaming, ill get to why in a sec, first i have to explain a game mechanic that enables online play in the first place

when beginning the game, you're actually hollow, i wont bother you with the lore if you actually wanna read but basically youre a walking rotting corpse, however you can undo this and restore your humanity by using

well

HUMANITY (an item),
becoming human actually has a LOT of benefits which basically make the game a cakewalk
for starters, you can now use another humanity stack to get 5 more estus flask (basically the main healing item in the game), this makes for an insane difference early on because one estus flask basically fills up your hp and 10 of them will be pretty much enough for an entire beginning area

you'll also be able to summon npcs (as long as you're following their quest lines), such as solaire of astora, to help you with some bosses, summoning these npcs normally make the bosses the easiest thing in the fucking world (if, the boss youre fighting lets you summon an npc, that is)

of course, this would make dark souls pretty much easy as fuck and it wouldnt be "dark souls", so there's another ingredient

PVP.

as long as you stay human, other people can invade your playthrough as phantoms and fight the shit out of you

if you win, youre free to go, however if you lose, you die and have to go back to the place where you died to not lose hours of progress

this is essentially insane because a good player could parry you and do a riposte, or get behind you and do a backstab, both of which erase through your hp no matter where you are in the game

the only plus of getting invaded is that no one 10+ levels above you can invade you, so invasions are actually pretty fair game, and if you win you get to absorb their humanity, which starts the cycle yet again.

another form of multiplayer is co-op, you can actually summon people into YOUR world on purpose so that they may help you with a bossfight or just to get through a zone