My first mutant creation, input requested

I'd recommend the tentacles have a paralytic effect, so anything hit by them won't be able to fight as much. Make it some sort of mushy growths that stretch out in different directions around it, and the tentacles are fired from them.  Similar to jellyfish stingers, where they grow, expel into the target, but this would either slowly retract them to 'reload', or just grow a new tentacle from that location (like fingernails or a lizard's tail).

You could even add a sort of intelligence to it so it only fires the tentacles if the pressure or sounds are within range but it confirms that they are getting farther away. This way the PCs can actually camp all night on the cap, search it for junk, but as they try to get away in the morning it starts doing the tentacles. If they just wander around it for a while or walk really slow, it never notices enough change to tell it that the PCs are escaping and will let them leave.

Since the creature 'hunts' based on vibrations, does that mean dropping an explosive on/near it would cause it to be deafened for a while?  Or dropping/throwing a tree trunk into the water to help distract?

The only problem is that the tree trunk can be eventually digested and will make the Pit fiend stronger in time.

For the larva, have it be a 2-part process.  The basic part is the larva itself, the rest of it makes the targeted individual think they are overheating and want to bathe in water.  Once they get in water the creature is put to sleep.  This provides a ready source of nutrients for the larva.

*UPDATE*

Pit Fiend
Defense Value: tentacles -40 / body +15
Endurance: core 240 / tentacles 50
Movement: nil
Initiative: +5
Attacks: 4-6 tentacles
Strike Value: 01-70
Damage: 2d10 slashing, + biotoxin 1

Strength: 220
Agility: nil
Accuracy: 25
Intelligence: 15
Willpower: 24
Perception: 100
Appearance: 5
Valuables: random
Experience: 00
Morale: nil
Size: 15m
Weight: 1000kg
Mutations: nil
Relics: 30%
Implants: nil

Creature resembling a giant fleshy greenish gray mushroom infesting large bodies of stagnant water. The stench can be overwhelming in enclosed spaces. Hazard checks might be required given the size of the area. Swamps or large bodies of water might have enough air movement to negate this. Building, rooms and pits would most certainly require it. This is up to the GM's discretion. It’s body consists of a thick tree trunk like anchor. It's “mushroom” top rests just below the water line and is covered by a thick layer of mucus which it uses the create a shell like covering of trash and junk. It is not uncommon to find trees, boards, vehicles and even bodies embedded in this layer. As such the crown is desensitized to all movement and PCs can actually stand and camp on it undetected. It feeds on bacteria created by raw sewage, dead bodies and decaying organic matter. Because it never moves it isn't uncommon to have flying creatures living in it's crown. This has no mouth or teeth, instead it eats through baleen much like a whale.

It “sees” through the use of thermal receptors (underground and cold locations only) and echo location like a bat or dolphin. A rhythmic, shallow deep thump can be heard up to 20 meters away. Throwing large object in the water will distracted it temporarily. Dropping an explosive device will “blind” it for 1d6 rounds. The Pit Fiend has 4-6 tentacles, each tipped with a 3 sided bony hook which grow out from below the water line from the trunk. Each tentacle is up to 40m in length which it uses to pull organic matter into the water and add objects to it's crown. Because it lives in absolute filth, the hooks are covered with a bio-toxin. Any successful attack must make a hazard check 1 or contract the toxin. (I really wanted some kind of disease but the rules only have 4 and I didn't feel like going through the work of making a new one) Because it feeds on anything it is unaffected by any toxins or chemicals. Fire however, if successfully landed on it's crown, does double damage for the first round. After that it will pull it's crown below water to extinguish the flames for 2 rounds. If PCs manage to set it on fire 3 times in secession it will retreat under water and disengage from combat, nursing it's wounds.

Once a year a special tentacle will grow with a hollow spear instead of a hook. It will try and harpoon a passing creature with this injector. On a successful strike it injects a larva which will gestate in 10 days to approximately 3cm. Along with the larvae it also injects a paralytic agent removing the target from combat. After this 10 day period the larva will release a mild nerve agent and the infected will severely overheated and seek to submerge itself to cool off. Save verses willpower. On a failed save the creature will submerge itself. When water comes in contact with the wound, the larva releases a sleep agent, drowning the victim becoming the first food source for the new Pit Fiend. The larvae is resistant to all chemical treatments, it can only be removed through surgical procedure. As it stays just below the skin surface so it is not difficult to remove and requires minimal skill.