Swamp Jellys are enemies in Demon's Souls.
They are giant, grey-colored jellyfish with barbed stingers that live in the swamp of the Valley of Defilement at the Leechmonger Archstone. They are one of the limited enemies that has a finite spawn number, only spawning once per playthrough.
Strategy
The player is entirely safe from them if they stay out of the water, as the Swamp Jellyfish cannot come onto land and will instead surround the land the player is standing on. From here they can be dealt with easily using ranged combat. If the player must engage them in melee combat, the Swamp Jellies possess only one attack, in which they will thrust out all their stingers in all directions in a short range around them. Alternatively, should the player not wish to engage them at all, these enemies move very slowly and one could just bypass them quite easily.