Moonshadow Grendels are a genetic Grendel breed done by Grendel Man. In all their incarnations, they are able to turn themselves invisible, with the help of an agent.
Version 1 of the Moonshadow Grendels is available from Creatures @ CU7. The way these grendels disappear from creature's sights is via Marcus K.'s Invisibility agent, included with his Ghost/Phantom Norns, and they turn invisible when very angry. Besides that, the only real differences between them and their Banshee ancestors are calmer temperament (although they still have a tendency to anger quickly), and a less of a need to eat.
A slight bug with these, which will be fixed in V2, involves the grendels unable to turn their invisibility off once it is turned on.
Version 2 of the Moonshadow Grendels is currently being worked on. They become invisible with an invisibility agent Grendel Man himself made. While it doesn't actually make the grendel itself disappear from complete sight, it can be seen if the agent is working by turning agent help on, and right clicking on the grendel in question. If the image showing the grendel in-game shows nothing, the agent is taking effect on the grendel.
Improving on V1, V2 Moonshadows will return to visibility whenever they are ready to breed. In addition, they can also be brought back to visibility via a tickle.
Other than invisibility, V2 Moonshadow Grendels have more additional edits than V1 did. While they have the temperament of their V1 counterparts, and have less of a need for food, they also have the CFE brain edits, and they will not breed/be fertile unless they are placed in a room lacking light. They can eat anything, and can get the same amount of nutrients from all types of food, be it seed, critter, or detritus.