| "The higher this attribute, the closer you are to beasthood when temporary transformed."
Beasthood is a stat in Bloodborne; it determines your maximum physical damage multiplier while using a Beast Blood Pellet or the Beast Claw. A higher Beasthood attribute gives a longer gauge when active, and as the gauge is filled your physical damage will slowly rise but your defense will also decrease. Beasthood will only increase physical damage, and does not boost arcane, fire, bolt, or blood damage. Beasthood build-up can be cleared by using a Sedative. This will instantly remove all effects of Beasthood, including the physical damage multipliers and defense reduction.
Beasthood has inverse scaling with your Insight, just like Frenzy. The more Insight you have the lower your Beasthood, and the less Insight you have the higher your Beasthood. It can be further boosted by attire and certain runes, and wearing attire with high Beasthood like the Ashen Hunter Garb or Brador's Set and equipping one or more of the Beast runes and the Beast's Embrace rune can make up for high Insight. Or it can let you run around with well over 600 Beasthood, which may be useful if you're using the Beast Claw and want its effects to last for several minutes before running out.
Simply having a high Beasthood stat is not enough to raise your attack. The Beasthood stat only determines the length of the "Transformation" gauge that appears when you use a Beast Blood Pellet or the Beast Claw. This gauge will slowly fill as you attack enemies with your weapon, and as the meter increases and passes certain breakpoints your weapon's physical damage will also increase, but your defense will likewise decrease.
0 Beasthood: no change to physical damage and defense
1 Beasthood : +20% phys damage, -20% all defenses
25 Beasthood: +30% phys damage, -30% all defenses
50 Beasthood: +40% phys damage, -40% all defenses
100 Beasthood: +50% phys damage, -50% all defenses
200 Beasthood: +60% phys damage, -60% all defenses
300 Beasthood: +70% phys damage, -80% all defenses
Note that the meter does not need to be completely filled before these changes take place. By simply having that much Beasthood in the gauge, you already have the boosted attack and reduced defense. For example, if your character has 350 Beasthood and uses a Beast Blood Pellet, then attacks an enemy to raise the gauge by 10, your character will already have the +20% physical damage and -20% defense. By attacking the enemy more until the 25 breakpoint is passed, your character will then deal +30% more physical damage. If the meter falls to 24 or below, your character's damage multiplier will go back down to +20%, so in order to keep the +30% damage boost you have to continue attacking so the meter doesn't fall. As such, using Beasthood effectively requires a very aggressive play style and can be risky due to the reduced defenses on top of that.
The amount of Beasthood gained from attacks varies from weapon to weapon, what kind of attack you're using, and the enemy being attacked. As a general rule of thumb, larger weapons will generate more Beasthood than smaller weapons, and heavy attacks such as R2s and L2s will fill the gauge faster than R1 attacks. Only certain bosses will fill the Beasthood gauge noticeably faster than others when attacked, namely Father Gascoigne, The One Reborn, and Orphan of Kos. For example, a Ludwig's Holy Blade in greatsword form will build up more Beasthood with its R1s than a Threaded Cane does, and its L2 combo will fill the gauge even faster still. If Ludwig's Holy Blade is used against Orphan of Kos, the 300 Beasthood breakpoint can be easily passed in a matter of seconds. Alternatively, since heavy attacks will fill a Beasthood meter faster than R1 attacks, it is also possible to rapidly fill it by using the Whirligig Saw's L2 and stunlocking an enemy until your meter reaches the +70% damage mark.
It is also worth noting that visceral attacks count as physical damage, and Beasthood will boost their damage as well. With high Skill, multiple Clawmark runes, high-grade physical gems, and a gauge with over 300 Beasthood, it is possible to deal a visceral attack with five digits of damage. That's enough to one-shot the Moon Presence, and most Chalice Dungeon bosses besides.
Beast Blood Pellets will activate the Beasthood gauge and allow you to build up the meter with any weapon, but they only have a duration of one minute, after which the meter will completely disappear and all damage boosts are immediately lost. For this reason, there is no need to have more than 350 Beasthood, as 300 is the final breakpoint and the extra 50 will let the +70% boost last for longer if you need to dodge or evade instead of attack. On the plus side, a Beast Blood Pellet will allow you to thoroughly abuse large weapons with high physical damage, such as Ludwig's Holy Blade and the Kirkhammer, and it's not uncommon for these weapons to deal 1,500 damage per hit against bosses when you have 300 or more Beasthood built up.
The Beast Claw weapon will activate the Beasthood gauge indefinitely when used in trick mode, but only attacks with its trick mode will build up the meter. Any Beasthood built up this way will not disappear if you switch weapons and the damage boost will carry over, however the new weapon will not be able to build the meter with its attacks and it will slowly empty until it disappears. For this reason, having a Beasthood stat of 600 or more can be beneficial as it allows the maximum damage boost to last for several minutes before you need to switch back to the Beast Claw and build it up again. Equipping the Beast's Embrace Oath Rune while using the Beast Claw will change its moveset and allow Beasthood buildup in both its normal and trick mode forms, and increase the amount of Beasthood gained in its trick mode. Unfortunately, this prevents you from using other Oath Runes such as Corruption or Impurity.