Personally I started with King's Field
It is nothing like Demon's Souls. King's Field died.
You have no idea how many things from Souls were introduced in King's Field
There is nothing in Demon's Souls that can be connected to KF beyond the most superficial loose bullshit like "dark" "fantasy" "dungeon" and such. Obviously there's a connection because of the company but the games are nothing a like. Demon's Souls invented a ton of incredible shit that people are now bitching about every day. KF didn't invent
anything.
Illusory walls, protostamina system, the need to actually read enemy moves in order to attack in time instead of flailing your weapon like in Daggerfall/Ultime Underworld, proto estus flask system where you could manually feel your flasks (which you first have to find in the world or craft, and you can feel it with different magical liquids, can even fill it with poison in KF4, so Estus system is a stripped down version of that), teleportation between checkpoints (again, Demon's Souls starting level teleportation and Dark Souls's bonfire teleportation feels like a very streamlined and stripped down version of King's Field fast travel), respawning enemies, random enemy drops that can change the way you play, durability system (and katanas, although powerful, are very brittle and need to be repaired frequently, thank god we don't have to wait realtime minutes in order to repair equipment in Souls games, a streamlined system that I actually like, along with upgrading weapons that also needs a material, like in Souls, but also takes place in realtime), player respawn, which lets you to die but the world doesn't reset to the state of the previous save (but it's limited and you must to have a special item in the inventory, another steamlining that I like in Souls), fucking item descriptions that tell more about the world (along with enemy and location description you could get in King's Field 3). Granted, none of it was invented in King's Field first, but it was first made in King's Field by FromSoftware and these games affected the creation of Demon's Souls and still affects them. And yes, I'm not even talking about same imagery about dungeons, same looking enemies, names, tropes and stuff, only focus on mechanics and game design here.