Also, I've read that you're an experienced GM, and you usually run deadly but fair games. I want this to run like Bloodborne or Dark Souls, or one of the many rogue likes out there. Advice?
Character death is a very tricky and specific element. I do not recommend it unless you are confident in your ability to GM to start with. If you lack the right judgement, communication and people management skills, you're going to have a bad time.
The first thing you want to do is communicate the possibility of player death. Preferably very big, so that people know what they're getting into. People are often attached to their characters and killing them, well, doesn't feel right for many of them. If they're not explicitly informed it will extra-suck.
Now that is clear, Including character death is, in it's very essence, removing plot armour. Understand the effect this has on your players. To keep their characters alive they have to balance risk of death versus completing their objectives. Very often, the objective suffers because of this sense of self-preservation. This is why, personally, I try not to be as punishing. That is personal preference, however.
To make it feel fair, you really have to establish the foreshadowing hard. This is not easy to do with text. While with Dark Souls you can carefully look behind a corner. In an RP you don't want to simply state "There was a narrow passage that took a sharp turn near the end." all the time. I mean once or twice is fine, but if you get too into it you will get posts of people carefully checking the narrow passages all the time. That's a lot more boring in RP's than videogames. Also, to make this actually feel dangerous and have players react properly, you need to establish a hostile atmosphere before throwing in the turn in the passageway. You have to both foreshadow and set an atmosphere before attempting these things to make them feel fair.
Lastly there's punishment. This is a beast all it's own. I touched upon this earlier, but how hardcore do you punish your players? In the end, it means you will be forcing hits and a lot of players are not comfortable with that because they consider it godmoding (again, communicate). If you go down the road of forcing hits, you better be good and confident in your ability and judgement.
There's two types of play that can invite punishment. Taking risks and bad decisions. Risk vs. reward can, IMO, best be handled by dice. This shifts the responsibility from you as a person to the random factor. Secondly, a dice roll can be used to determine the severity of the punishment. That means that this failed ambush doesn't automatically kill the player, but it's possible they get captured, injured, etcetera. Giving other players an incentive to act/not act. I'll get back to the effect of punishment on other players in a minute.
Bad decisions... Well as a GM you will see these a lot. Some of them are character-driven, but a lot of them are player-driven. Hey, let's charge that line of pikes head-on and also solo. We know there's a sniper out there, but maybe if I run really fast I can cross those 2 miles without getting shot. Maybe these bloodthirsty orcs will listen to me if I go as an unarmed convoy even though I personally killed all of their mothers. Players make a lot of bad decisions and the worst part is; they're often unaware of this. Dice can come to the rescue again, though less favourable I guess, but as the game progresses they're supposed to learn from them, be it on their current or next character (always allow people to make new characters if their current characters die). However, this means that bad decisions should be, when looked at logically,
unquestionably be bad decisions. So again, foreshadow hard and communicate limits of characters and enemies.
Lastly lastly, we have the impact of punishment on players that aren't directly punished. One person getting punished shouldn't make an objective impossible. This can be tricky in a ton of situations. Like, don't do stealth missions. Stealth missions are horrible if only for this reason. But if the one ambush guy goes in solo, he also alerts the enemy of the presence of your group. Maybe you picked a specific strategic position they now have the opportunity to flank. Etcetera. This is really tricky and requires a lot of thought of you as a GM. Punishment may make a scenario more difficult but it should by no means mean the objective or survival of other players are now doomed.
So, that should cover most of it. Also rogue-likes are terrible examples of what you're going for ;p