At low tide, the moonless cross into our world, emerging from the ocean to find and attach to hosts. While they might possess animals, they prefer to attach to humans. At first, the moonless have only subtle influence over their host, but their control grows more and more complete over time as they alter the host’s mind to become compatible.
The moonless are intelligent, though their alien way of thinking can be hard to understand. They communicate with one another in their own language, and they seem to have access to all the knowledge of their host.
Moonless are invisible and intangible to most, but luminists can crack pearls to see and fight them. To a luminist, moonless have a translucent, ectoplasmic appearance, through which some of their solid organs are visible. Moonless are always physically attached to their host in some way, and they come in all shapes and sizes. Some moonless are small enough to hide from luminists beneath clothing or within walls.
Only death can separate the moonless from their hosts - ideally the death of the moonless, not of the host. When moonless die, they weep tears that crystallize into new pearls.