Algoid
Detail of "Mermay Day 19: Algae" © ArtStation user "pepetheimpanzee", accessed at his gallery here
[The algoid is a Fiend Folio monster with interesting abilities but no clear world building hook. Why does algae assemble into a humanoid form? Why does it attack people? Why is it chaotic neutral as opposed to any other alignment? Why does it have psychic powers? The 2e version doesn't address those questions, which might be why the algoid morphed into the vine horror in 3e and became one of the blights of 4e onward. Those at least are solidly evil and hate people, so have a reason to attack adventurers. I'm going back to the original and tying it into some of Paizo's occult concepts, and downplaying its ability to animate plants a bit. The 1e algoid can animate trees with the strength of a treant. That's a very powerful ability on a treant, let alone a monster with half the Hit Dice. That is one of the things I like about 5e's approach to monsters; giving animated trees their own, and much weaker than a full treant, stat block.
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Algoid CR 5 CN Plant This green humanoid figure has underdeveloped features with little more than a pair of eyes and a suggestion of a face. Its arms and legs widen towards the hands and feet, as if it is difficult to keep them solid.
When ley lines pass through swampy environments, their occult energy can resonate inside one of the most mundane of all organisms: pond scum. Algoids spend most of their lives discorporated into a mass of algae and water plants, peacefully photosynthesizing and absorbing nutrients. If creatures attempt to eat them or drain the water in which they dwell, or if another creature attempts to tap into the power of their ley line, the algoids will rouse themselves to attack. Algoids can animate mundane plants to strike at enemies and stun creatures with pulses of occult might, but their most dependable tactic is simply to lash out with blobby fists. Algoids are sodden with water, which protects them from energy and physical attacks alike, but renders them vulnerable to spells and effects that manipulate water.
Algoids are not evil and can be negotiated with, but they are also quite stupid and have simple needs. A creature wishing to attune itself to a ley line without angering its algoid denizens may be able to assuage them by offering up interesting trace nutrients or by killing or driving away a particularly obstinate herbivore. Algoids communicate through telepathy, typically simple phrases like “go away”, “kill elephant” or “sunny days good”. They can also communicate with mundane plant life in their habitats, and sometimes have bits of gossip to share, although they are much more interested in weather patterns and soil quality than the concerns of humanoids.
