There is a food chain of decay. A sun-linked chain made by plants. A red chain of animal flesh. A dark chain of dead and dying things. The doomed feasting on the ruined who ate the dead. A pyramid of ghouls. Crocodiles survive every extinction. They are a final link of the negative chain. Slow. Patient. Waiting for the world to make a mistake. The river-dwellers eat the things that ate the things that ate the world. They live on.
And the unusual character of the Crocodile, and it's tidal metabolic tick, lend it strength.
There is a fungal life that rules the flesh. Consuming insects, crabs and shrimp from the inside. It saw the crocodile and struck. It found the target lodged. The slow churn of the crocodiles flesh left no purchase for infection. It's infinite calm and brass-cogged cogitatative mind refused the touch of fear. The fungus was trapped. If could only infect the crocodile to a limited extent, could never consume it totally. The crocodile was likewise pinned. It could never fully eradicate the fungal life wrapped around it's scales and mind. Perhaps the only creature alive that would not go irrevocably insane when being consumed by a semi-intelligent fungi.
They learned to live together. They became something new.
The Fungus warms the crocodile from within, and cuts the rope that binds it to the sun. It fixes flesh and lends long life. In return it rides inside a king. A massive, close-to-deathless deep-dwelling apex-killer.
Not a king actually, an Ambassador.
For several excellent reasons.
One - They speak fluent Myconid and almost nothing else does.
Two - The endless conversation between fungal dream-state and crocodile-brain lends them a talent for other languages.
Three – Connection to no particular racial or political group.
Four – Utterly reliable, impossible to bribe.
Five – Cold, quiet crocodile minds have a flair for Realpolitik.
Six – They are their own bodyguard, a regenerating quasi-psychic crocodile the size of a truck is difficult to scare.
Seven – They seem to love the work.
Eight – even though they are the living expression of a continual conflict/symbiosis between a near-immortal reptilian mind and an almost-alien fungal intelligence, they are actually less insane than most of the things they represent.
Intelligence. Calm. Probity. Reliability. Subtlety. Immense patience. The fact that its a fucking tank with teeth. Gigantic fungal Crocodiles are the Ambassadors of the Underdark.
And the embassies. Inside the Crocodiles mouth (or stomach) is sovereign territory. A large male can carry three to five adults in its mouth in reasonable discomfort. Like a close cab. The passengers nest like baby gators. You will be utterly safe within. (Diplomatic immunity.) Unless the political situation outside changes. Should this happen, you will be swallowed. Though it will take some time for you to be digested.
Inside the mouth, things are usually calm. Politically important dignitaries can be carried through the darkest war zones in sticky comfort.
Two voices. One, deep, low, sardonic and bass. The other tiny, fast, verbose, polyphonic and irreverent. But one mind. One would quite like to eat you, the other to infect you. Though they will only express this wish in the most charming Wildeian small-talk.
It is possible to persuade one voice of an Ambassodile to hide something from, or deceive the other. But only so long as the silent overmind finds it amusing.
Mad fungal moths sometimes perch in its back in rows like teeth-cleaning birds. They are messengers, flapping wetly through the empty caves to whisper political secrets in the Crocodiles ear.
Its gut holds secrets and gold. Bits of treasure here and there, but also secrets and incriminating works. The Underdark equivalent f the Nixon tapes are probably inside a Crocodiles stomach somewhere.
Should the PC's actually kill one (or more likely find one dead) they will be in much more danger from the secrets they find in it's belly than they were from the creature itself. Like a trippy conspiracy movie.