I am but a humble Shrimp.

[Dragon Lord]

Whoops, I didn't realize that I had forgotten to reply to this topic.

The boss I'm talking about in Demon's Souls in the first one -- the Vanguard Demon. If you die to him, you just continue with the cutscene of the Maiden in Black summoning your soul in to the Nexus. However, if you defeat it, you get transported to a bonus area that has a bunch of items you can pick up, mainly consisting of a good heap of upgrade materials, and then you proceed into a cutscene where you cannot avoid being killed by the Dragon God, putting you back on track with the Maiden summoning your soul into the Nexus.

I think I'd have to say that the "hardest" opening boss fight in a Souls game is probably Dark Souls 3, because that's the only one where you're actually put up against a boss with no advantages. For example: In Dark Souls, if you follow the intended route, you avoid fighting the Asylum Demon right away, go around and up the Asylum, get your weapon, and then go back into the boss room on top of a ledge, where you can do a plunging attack into the Asylum Demon for a huge chunk of damage (nearly 50% of its HP), giving you a gigantic advantage in that fight.

Dark Souls 2 has no opening boss fight, and you're just free to explore one of two areas where you can gain souls, get some levels/equipment, and then go into one of the boss fights with some muscle to back you up. Doesn't help that both of the first bosses you can fight in Dark Souls 2 are relatively easy. You can even cheese one of them into doing its dash attack off the edge of the arena to end the battle instantly.

Bloodborne has the Cleric Beast as the intended opening boss fight, and the Cleric Beast is in no way dangerous. It just has a whole hell of a lot of Health for an early game boss, making it feel dangerous. Otherwise its attacks are super easy to dodge and it's just a matter of chipping away at all of that HP. Father G is pretty easy too since you can Power Attack > Backstab chain him pretty easily. The only time he gets slightly dangerous is after his transformation, but you can just parry him at that point iirc and his attacks are pretty easy to read for parrying. I'd say that Bloodstarved Beast makes a harder early game boss than either the Cleric Beast or Father G. Especially when Bloodstarved hits you to put you at like 5 HP and poisons you at the same time, resulting in an unavoidable death. Plus he's much more aggressive than either of those two are most of the time.

I don't know about Sekiro, since I still haven't played it yet, but I will definitely let you know if I think that it has a harder opening than any of the Souls games or Bloodborne once I do.

Here's to hoping that Elden Ring has a brutal opening that bends every player over the table and... well, you get the rest.