Lamia: This is a new monster, a desert-dweller that has the torso of a woman and the lower body of some sort of beast. The illustration depicts the lower body as that of a lion, but it’s not specifically said in the text, so it could really be any animal. As is the way of things in D&D, the illustration shown here becomes the default depiction for the monster in the future.
Although they have quite a lot of Hit Dice, they seem to rely more on guile and trickery than brute force. They can cast charm person, mirror image, suggestion and create illusions, all spells that lend themselves to hoodwinking the PCs. They also have a touch that permanently drains wisdom, and that’s major in a game where ability score increases are hard to come by. If your wisdom is drained below 3, you become the lamia’s willing slave. Which is bad, because they like to drink the blood of their victims and then eat them.
Introducing lamias to the campaign shouldn’t be difficult, as their preferred desert climate is a fairly remote one.
Lammasu: These sphinx-like creatures first appeared in Supplement I. Their Hit Dice has increased from 6+2 to 7+7. Their alignment was Lawful, and is now Lawful Good, and they are still very helpful to characters that share their alignment. They also get a 30% magic resistance that they did not have before. They can still become invisible and cast dimension door. Their aura of protection from evil is now twice as potent as it was before. They also still cast spells as a 6th level cleric, and it’s spelled out here exactly how many spells they get a day; they now get 2 extra spells of first level, and 1 extra second and third level spell. Any cure wounds spell they cast is now twice as effective as before. There’s also now a 10% chance that any lammasu can cast holy word. In OD&D they were said to be able to speak any Lawful or Neutral tongue. Now they just get their native language and their alignment language, as well as some limited telepathy.
Lamprey: Lampreys first appeared in Supplement II, and they get a fairly extensive rewrite. The original lamprey was a 3 Hit Dice creature whose blood drain ability was poorly worded; they drained “one level per hit point” which could be interpreted in all sorts of absurd ways.
In AD&D, there are normal lampreys and giant lampreys. Both are much the same, only the giant variety has more hit points. Their blood drain is much more reasonable, being 2 hit points per round for each of its Hit Dice (meaning a normal lamprey drains 2 hit points a round, and the giant kind drains 10 per round). Simple, and no way to interpret it as level drain. Everyone’s happy, except for super-sadistic DMs.
Larva: Larva are a new monster. They are the souls of the most selfish and evil beings, that dwell in Hades in the form of human-headed worms. Night hags use them to bargain with demons and devils, as they are useful in creating imps and quasits. They’re also used by liches to maintain their undead status.
For outer planar creatures, these are surprisingly weak. They would make a decent encounter for low-level PCs, but it’s extremely unlikely you’d ever find one outside of the planes. They seem to be included more for flavour than anything, although any creature used for bargaining can be quite easily turned into a plot hook. Imagine the PCs in a race against some demons to find a “great treasure”, only to find at the end a big pit full of these things. I don’t see them as a monster you could use very often, though.
Giant Leech: These things first appeared in OD&D Vol. 3, and were expanded on in Supplement II. They’ve changed a lot. Number Appearing has increased from 2-12 to 4-16. Armor Class has worsened from 8 to 9. They’re slower, their speed having dropped from 6” to 3”. They used to have 2 Hit Dice, but now they range from 1 to 4. Their bite damage has been majorly nerfed from 2-12 to 1-4. The only other major change is with their blood drain ability. In OD&D they drained levels at the rate of one per turn. Now they drain hit points at the rate of 1 per round for each of the leech’s Hit Dice. It doesn’t sound much, but while the victim is still in water it’s very unlikely they’ll notice until half of their hit points are gone. They still cause disease, but they aren’t quite as deadly. In OD&D, the disease was automatic, and killed you in a month. Now there’s a 50/50 chance that the leech is diseased, and it is fatal after 2-5 weeks.