This is a proper spoiler for the Blood Cave so I never get a PM about it again, ever. I apologize in advance for some gaps in the skill information when it comes to the monster guide at the end; I don't have access to all my info, so I'm doing this mostly off memory. I also apologize for the low-quality, and low texture quality of the screenshots--they were taken a long time ago, and from the editor.
The entrance to the Blood Cave is located in Act 3, at the Tsongmo Peak, between the Eastern Silk Road, and the Western Silk road waypoints.
This image (from the gamebanshee walkthrough), shows the area where the Blood Cave is found.
M is Gulga, the 2nd Occultist Vendor.
1a is the entrance to a cave with some spirits in it (for the Mystery of the Mountain sidequest). In that cave, to the left, is a winding passage through some stalagmites that leads to the "Mysterious Passage."
This is a short, narrow passageway littered with spiders that leads to the Blood Cave itself. At the end of the Mysterious Passage you'll find a decapitated body and a quest object on the ground. (See the Adventurer's Wife spoiler for more info about that).
Warning: if you actually read the tattered parchment you pick up, it will dramatically advise you to turn around. This isn't just flavor text: the Blood Cave, and the areas beyond are relentless and grueling. The amount of money you can make running the Blood Cave is pretty big, don't get me wrong, and there's more experience in there than in any other equal-sized section of the game, BUT, you will have to work for it. Most players will advise you to wait until you're a much higher level and then return to the Blood Cave. If you do decide to run the Blood Cave, and what follows, as soon as you find it, then you should know that you're in for a long and bloody battle. By the time you defeat the Blood Cave, you will be a changed person. Returning to the path to fight Tigerman will seem insignificant. You will be like a war veteran who's come home to run a breakfast diner. It's just not the same.
This is a large, fairly-open, and barren cavern populated mostly by blood demons, and a few Blood Cult monsters--spellcasters who summon more blood demons and debuff you.
You'll find the cave itself dissected with an underground river. Tall, immobile blood vessles litter the edge of the river. Most of them are just background decorations, but some of them--who appear slightly brighter than the rest--will explode if you get too close to them. They deal a lot of physical, bleeding, and vitality damage. They also have a small chance to stun you. You cannot attack them directly, but you can trigger them with summons.
There are two bridges across the river. The second will be blocked by a lot of these exploding blood vessles. You will need to figure out a way to clear them in order to cross. Doing so will lead to a mini boss, the High Priest. He's kind of a dick who summons Blade Dancers.
Once you've killed the High Priest, you'll be able to open the door to the Temple of Eternal Love.
Note: this is sort of the point of no return. Once you pass into the temple, you will not be able to return to the bloodcave, and you will hit the rebirth fountain there. In order to leave now, you'd have to portal back to the Eastern Silk Road. The only other way out is to go forward until the end, at which point you will be portaled back to Gulga, the 2nd Occultist vendor.
Temple of Eternal Love
This is a smaller area, with filled mostly with Sileni and some Blood Cult. The Sileni are like uber Satyrs--they're fast, tough, melee monsters. You'll run into a few more Disciples here too.
At the end of the temple is Aegobolus, another mini boss. He's not a difficult boss if you manage to clear the area before fighting him. Just don't make the mistake of running into one of the rooms to his left or right, as there's a small clutch of Seductresses in each one--you definitely don't want them involved.
Once you beat him, you can pass through another ichy door, on the other-side of which will be a portal that will take you to the Sanctuary of the Bloodborne.
As with the passage to the Blood Cave, once you pass through to the sanctuary, you will not be able to return to the Temple.
Sanctuary of the Bloodborne
This is the longest, and most grueling section yet. There are huge amounts of Blood Cult, and a new monster, the Reaver, who will make you scream at your monitor.
Making it through will be a colossal feat of dedication. At the end, you'll see a large chest standing between two bloody pillars. The chest is a trap, and stepping directly toward it will port you to the final boss fight with Leinth. The chest itself will fade away as you approach it, and doing so will spawn Blade Dancers all around you. You can either fight them there, or ditch them by running ahead into the boss room.
Doing so will pop some familiar crystals behind you to block your exit.
Getting to Leinth is far harder than defeating her. Avoid the mobile projectile she summons, and don't get surrounded with the tormented souls, and you should be fine.
Once you defeat her, a vortex portal will spawn that will take you back to Gulga, where you can sell your loot for a higher-than-normal return.
All blood cave monsters have good bleeding and vitality resistances, and all deal some amount of bleeding damage with their attacks.
from left to right: Blood Taint, Blood Thirster, Mutilator, Blood Beast, Blood Beast (Champion).
The blood demons, known as Malefice in game. Race: Demon.
The little guys deal physical and bleeding damage. They have high bleeding resist, and decent vitality resist. The mutilator deals some hefty damage, but otherwise they're not that hard to deal with.
The blood beasts are another story. The damage they deal is pretty huge, but their puke is the real problem. They won't puke on you if you're in melee range, but if you're farther away they will, and it deals some massive damage, and slows. Unlike other breath attacks from other monsters, this projectile hits far more reliably.
from left to right: Acolyte, Seductress, Disciple
These are the basic Blood Cult monsters you'll run across. Race: Damned.
The Acolyte summons small blood demons, and has a projectile attack that deals bleeding damage.
The Seductress has a short-ranged aura that will reduce your damage output, and convert your summons. She can also summon a cluster of 3 blood demons that are different than the others; you'll notice that they have dark, almost black skin. If you let her summon another cluster, things get a little more complicated. If 5 or more are in range of the Seductress, they'll be able to fire a synergy attack (like the albino spiders in Act 4). If allowed to charge their synergy attack fully, one of the blood demons will explode, dealing damage and disrupting you. Fighting more than one Seductress can obviously cause massive problems, as these summons can synergize off the summons of another Seductress, and with no delay. Let them accumulate, and they will be exploding like crazy.
The Disciple can summon a Blood Beast. He can also cast a blood rain that will reduce your resistances and deal bleeding damage to you. He can also fire a special attack from his eyes that will deal a good amount of damage and disrupt you for a long time. The special eye attack has a long cooldown, a very obvious animation, and the projectile is slow, so it's not hard to avoid.
The Disciple also always equips a vitality staff, and vitality staves are worth bank, so rolling these guys can be pretty damn profitable.
The mini-boss at the end of the Blood Cave. This guy is like a Disciple on steroids, but instead of a Blood Beast, he summons ethereal, Melinoe Blade Dancers. And they hurt, a lot.
left to right: Carver, Impaler, Butcher, Aegobolus (back)
These are the Sileni. Race: Beastmen
These Uber Satyrs are much faster and deadlier than the cute little fuzzy ones you faced in Act I. These guys equip some serious gear, and have a small array of melee skills at their disposal. Watch out for the head butts.
Aegobolus is the mini boss that has a number of attacks, several without projectiles. However, he's a push-over once you clear away his friends.
Aegobolus has a small chance to drop that staff he's holding. It's an Ethereal Staff of the Magi, a higher-level, and vitality-based version of the Staff of the Magi. It's pretty badass, actually. His chance to drop it is about 1 in 25.
left to right: Reaver, Reaver in Demon Form
The butt-rockers of the Blood Cult. Race: Damned.
Only found in the Sanctuary of the Bloodborne, these little asskickers have a bolt-style attack that disrupts you and slows you by a massive amount. There's no way to avoid this disable. She likes to use this to make you eat her second attack, which is a slow-moving wave attack that looks like black-tailing flames. It also disrupts, as well as dealing a lot of damage (a portion of it fire damage). But, even if she doesn't blast her wave, her bolt disable can often force you to eat Blood Beast puke or Disciple blood rain. Never rush into any Blood Cult mob with a Reaver.
Another problem with Reavers is that when they get very low on life, the transform into an ethereal, winged demon. These things hit hard, and it's hard to dodge their attacks. However, once the Reaver does transform, so does her race, from Damned to Demon, so racial bonuses will help against her in this second form.
Reavers equip wands, which are a unique weapon type that fire a ranged, vitality-damage projectile and are only a one-hand weapon. The downside is they can't be used with any weapon attacks. Reavers also have a chance to equip an MI version of this weapon, and a small chance to equip a unique wand. They are the only source of wands in the entire game.
This is the main boss, found at the end of the Sanctuary of the Bloodborne.
Leinth has a basic, blood-based projectile. She also has heat-seeking projectile that will follow you around for a short duration. If it gets close enough to you (or runs out of time) it explodes. She will also summon her tormented souls, who are slow moving, but painful if they get the surround on you.