Kagedama

Description
The Kagedama is a mushi that resembles a translucent snake-like shadow. It resides in the shadows of old trees. When animals and people rest in the tree's shade, it enters through the ear of its chosen host and embeds itself in the brain. There it begins to consume memories, apparently in no particular order, though memories necessary for the host to function are likely left until last in order to prolong the host's lifespan. In addition, the host is inflicted with insomnia except for very brief moments when the Kagedama splits itself and sends its offspring out of the host to take up residence in the shade of a nearby tree.

Weakness
Sunlight. However, once the mushi has embedded itself in a living host, it cannot be removed.