This might count as a thread tax, but play any of the clones. I have a soft spot for The Surge, but any do Dark Souls better than Dark Souls. Fuck, even some Resident Evil 2 is more Dark Souls than Dark Souls.
There are 3 elements to the appeal. Technically 4.
1. The interconnected world. Being lost, on your last sliver of health, searching desperately for a bonfire, only to kick a ladder or open a door that links back to a safe space and creates that "ah ha!" moment as the entire world fits together. Resident Evil does this.
2. The different builds. After a first playthrough, there's lots of things you missed, weapons you couldn't use, etc. You can immediately start theory crafting unique builds. This doesn't work however due to the difficulty. Play New Vegas with a perk every level mod to experience this kind of novelty build craft.
3. The sense of progression as you learn enemy pasterns and level up. The problem with Souls games specifically is that at a certain point you realise the best tactic is to run past everything. I recommend incremental games like Digseum or perhaps the recent "A game about digging a hole"
4. The main reason people claim to love it. It was the first mainstream game in a long time to challenge the player. Granted, it does this mostly by cheap shots and trial and error bullshit.