Lampreys are leech-like eels which inhabit both fresh and salt water:
They do not inhabit shallow bodies of water.

Drain Blood: They feed by biting their victims, fastening themselves to the hapless creatures with sphincter-like mouths ringed with cruel teeth.
Once attached the lamprey will begin to drain blood on the next and each successive melee round.
The rate of blood drain is equivalent to 2 hit points of damage per hit die the lamprey has.
Thus, a giant specimen will drain blood causing 10 hit points of damage per melee round.

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Lamprey, Normal

FREQUENCY: Uncommon

FREQUENCY: Very rare ([Cold Freshwater Surface])
FREQUENCY: Uncommon ([Cold Freshwater Depths])
FREQUENCY: Very rare ([Cold Saltwater Surface])
FREQUENCY: Very rare ([Cold Saltwater Depths])

FREQUENCY: Uncommon ([Temperate Freshwater Depths])
FREQUENCY: Very rare ([Temperate Freshwater Surface])
FREQUENCY: Very rare ([Temperate Saltwater Surface])
FREQUENCY: Uncommon ([Temperate Saltwater Depths])

FREQUENCY: Very rare ([Tropical Freshwater Surface])
FREQUENCY: Uncommon ([Tropical Freshwater Depths])
FREQUENCY: Very rare ([Tropical Saltwater Surface])
FREQUENCY: Uncommon ([Tropical Saltwater Depths])

NO. APPEARING: 1-2
ARMOR CLASS: 7
MOVE: 12"
HIT DICE: 1+2
% IN LAIR: Nil
TREASURETYPE: Nil
NO. OF ATTACKS: 1 ~ 18
DAMAGE/ATTACK: 1-2
SPECIAL ATTACKS: Drain blood
SPECIAL DEFENSES: Nil
INTELLIGENCE: Non-
ALIGNMENT: Neutral
SIZE: S
LEVEL/X.P. VALUE: II | 28 + 2

Lamprey, Giant

FREQUENCY: Rare

FREQUENCY: Rare ([Temperate Freshwater Surface], [Temperate Freshwater Depths])
FREQUENCY: Very rare ([Temperate Saltwater Surface])

FREQUENCY: Very rare ([Tropical Freshwater Surface])
FREQUENCY: Rare ([Tropical Freshwater Depths])
FREQUENCY: Very rare ([Tropical Saltwater Surface])
FREQUENCY: Rare ([Tropical Saltwater Depths])

NO. APPEARING: 1-4
ARMOR CLASS: 6
MOVE: 9"
HIT DICE: 5
% IN LAIR: Nil
TREASURE TYPE: Nil
NO. OF ATTACKS: 1 ~ 15
DAMAGE/ATtACK: 1-6
SPECIAL ATTACKS: Drain blood
SPECIAL DEFENSES: Nil
INTELLIGENCE: Non-
ALIGNMENT: Neutral
SIZE: M
LEVEL/X.P. VALUE: IV | 165 + 5

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<note that there are 2 lamprey wound images in the MM>