Keeper (Babylon 5)

A Keeper is an organic construct used by the Drakh to control the behavior of an individual enemy. It is an octopus-like creature consisting of a small body, a few tentacles, and a single eye. It presumably derives sustenance by attaching itself to its host's circulatory system, but the details are not known.

Biology
A Keeper can control its host's behavior, but only when its own interests are at stake. Consequently, they can be tricked by convincing them that the host's actions are irrelevant to the plans of the Drakh. Londo discovered that he could put his to sleep by drinking enough alcohol, allowing him -- for a short time -- to take actions that would otherwise be prevented by the Keeper.

Like the Shadows, a Keeper is invisible in normal light. They become visible, however, if injured or rendered unconscious.

Even if seriously damaged, a Keeper will regrow on its host in a matter of minutes.