Bloodborne's bosses are getting tougher now. Your third foe is the Blood-starved Beast.
Bloodborne's been throwing Antidotes at you for a while now, and you've probably been wondering why. This is why: make sure you have them equipped in a quick slot before entering the boss arena. Watch out for the liquid coming out of the Blood-starved Beast - more in the latter phase of the battle - as it will poison you severely.
Poison build ups with exposure, and drains away over time, so if you are exposed you can survive by avoiding another dose for enough time for your buffer to drain away. Save Antidotes for when you really need them. Keep using Blood Vials for preference - they're far more plentiful. If your poison gauge hits 50%, it's probably time to use an Antidote. There are three more behind the statue in the arena, if you need them.
There are a couple of things you need to know about this fight. First, it's not required to finish the game, but speaking to the NPC in Hunter's Dream after killing it gives you access to a key optional Zone with some important items. Second, after fighting this boss you'll face an enemy called Death Dealers in some areas; read the section on how to escape the Unseen Village for more info on that.
How to beat the Blood-starved Beast
As for the fight itself: the Blood-starved Beast is always dancing out of reach, so use your longest weapon here; remember that it may be slower than the attack you're used to, and go for one or two hits where you might otherwise push for three or four.
In the first phase, your best bet is to get nice and close, and then as soon as you see the beast's forelimbs move, get behind it or to one side; behind works well, because you can get there in one dodge thanks to the creature's agile attacks, which usually carry it past you. You'll need to dash in and attack, as it'll be out of your range.
It's very important never to stand in front of the Beast, because it has a bite attack that inflicts Poison, which is a massive pain. Apart from this, the mid-range attack is a series of rapid slashes, requiring a couple of dodges to avoid. That's why it's best to stay close, but not in front, and wait for its less dramatic single or double slashing attacks.
If you're a long way away, you'll have to dodge a leap attack, but this is actually a great opportunity for you. Roll so you'll land behind it, and there's enough time to unleash a Charged Attack and Backstrike. This isn't easy because of how much the Beast moves, so if you're struggling just do a normal attack instead.
All well and good, but when you have smacked him down to half strength the Blood Starved Beast transforms. Well, I say transforms; it stands up on its hind legs. Run away. The Beast will push out a gas which will knock you down and possibly poison you.
During this phase of the battle it's imperative you back away whenever you get poisoned and wait for it to dissipate. You will get poisoned pretty much every time you attack, unfortunately, so this is gonna drag on a bit.
Just be patient: attack once or twice, back off and wait, and go in again. This is a war of attrition and the grimmest survivor will win. I wouldn't risk a Charged Attack at this stage; it moves too quickly and too strangely. Just whittle away with normal attacks.
If you have a collection of Pungent Blood Cocktails, they're very effective on the Beast, allowing you to perform Visceral Attacks with ease. Here's a demonstration by Flatts:
After you've defeated the Blood-starved Beast, light the Church of the Good Chalice lantern.
Hooray! We're off to the Healing Church Workshop.