Dark Sargassum

A Dark Sargassum is a Dark Young that has taken to living in water. They appear at first glance to be a floating mass of black, rotting seaweed.

In the RPG...

Dark Sargassum are much rarer than the Dark Young—which have become one of Call of Cthulhu's most common monsters—and also much bigger. An average Dark Young is 220 SIZ, while a Dark Sargassum is 425—so almost double. Despite getting 20 attacks per round, a Dark Sargassum can only make one attack per person. Each attack averages 120 damage though, so it's not like it needs any more. Even if the Keeper chooses to use the less lethal "grab and feed" option, the victim needs to oppose the Dark Sargassum's 220 STR to escape, which an average Investigator has a measly 6% chance of doing. Other Investigators can help out, to add up to two Bonus Dice, though again for the average Investigator, this only increases their chance of escape by 5%.

    The Dark Sargassum is immune to heat, blast, corrosion, electricity, and poison, and only takes 1 damage from firearms, so trying to kill one would be practically impossible. It does take normal damage from melee weapons, but good luck to any Investigator trying to club it to death.

    The 7e Malleus Monstrorum changes the POW and DEX of the Dark Sargassum. Neither's average is changed much, though POW's range is lessened, whereas DEX's range is increased. It appears that the only reason for the change is to match the POW and DEX of the Dark Young.

In a video game...

Being aquatic, I'm not sure where they would fit in a typical Call of Cthulhu story. Sure they could attack a ship that's being travelled on, but being do deadly and practically indestructible means that such an event would just be an elaborate death sequence. Perhaps they could just be used to advance the story, where said ship is destroyed and the players are stranded on a nearby island.

In a film...

I'd say a film would suffer the same problems as a video game in this instance. As a side note, the monster in Deep Rising is practically a Dark Sargassum, so if you're looking to see one on film, perhaps you should just seek that out.