Guardians of Crystallizers of Dreams

The Guardians of Crystallizers of Dreams—hereafter simply called Guardians—are dark, ghostly, floating, jellyfish-like creatures with glowing, yellow, cat-like eyes from a dimension between reality and sleep. As their name suggests, they guard the Crystallizers of Dreams from being used, which are yellow egg-shaped objects about a foot in diameter that have the ability to transport the consciousness of a sleeping person to far-off locations or dimensions.

In the RPG...

Every time someone uses a Crystallizer, there is a cumulative 1D10% chance that a Guardian (or pair of) will appear. They attack by entangling their vicim in their tentacles. The victim must make an opposed STR vs POW roll to escape else they are taken back to the Guardians' dimension and later found in the Waking World in a deep sleep from which they will never wake. For an average Investigator, breaking free has a 29% chance of success, though they have 3 rounds to achieve this, bringing the chance up to 64%, which isn't too bad. They have no armour, though being immaterial, can only take damage from magic that affects INT or POW. The only change I can see that the 7e Malleus Monstrorum has made to Guardians, is that their chance of attack has been bumped up by 10%.

In a video game...

As I've said before, I'm not a fan of magic in Call of Cthulhu, so I personally would never use a Guardian in a video game. Besides, the Crystallizer itself would be hard to implement in a game as each location would have to be created, so its number of uses would have to be limited anyway, negating any paranoia that the Guardians' appearance might instil.

In a film...

The Crystallizers and their Guardians could make for a pretty interesting storyline, though magic would probably have to be introduced if the protagonist had any chance of defeating them.