The symbiotic jelly exists by controlling the mind of another creature, which it then uses for protection and to obtain sustenance.
These creatures choose unoccupied caves in which to dwell, and stick to the ceiling in the shadows, where they are almost impossible to detect. When a carnivore ventures into the cave, the symbiotic jelly will attempt to use its innate charm monster ability to persuade the intruder telepathically to remain in the cave and attack any other creature which enters.
Thus, if a huge cave bear is charmed, it may be made to appear as a weak bear cub, while the back of the cave might appear to contain a rich vein of gold ore if a dwarf entered the cave, or a sumptuous banquet if a hungry halfling wandered by. Instinct: to feed of carnivorous instincts

• Read your desires
• provide what you want the most
• make its host appear harmless and weak