The voice didn't tell him to keep facing forward, so he turns around slowly.  The creature might be bluffing and relying on detect thoughts or similar divinations to probe his mind (after prompting him to think of secrets), so he starts thinking in Protean.  He doesn't actually have any proper spells he can cast swiftly, and he would have about a one in three chance of messing up the spell anyway even if he did, so his best option is acid if it comes down to combat, which it hopefully shouldn't.

He finishes turning, hand at his side, to show he isn't preparing a spell or anything.

A small creature, about a foot long (plus another foot of tail) is currently clinging to the ceiling, as if it were an arthropod several orders of magnitude lighter. It resembles a freakishly large rat, but with forepaws that resemble tiny humanoid hands and a pale, furless face that almost looks like a man's face in miniature, or perhaps a ghoul's.

Not a magical creature he has heard of before... which raises the odds it is some rare kind of creature with a unique language that will get him rejected by his god if he kills it!  So no pressure, just need to work out on option that keeps it alive (or allows Fernando to learn it's language first) and keeps the first mate happy.

The human face is a bit unsettling, but he could learn to tolerate it, even like it, if it meant 4th circle casting on his side.  Maybe he should offer to become it's familiar instead of the other way around?  Except he would need to be tinier than it...

He looks the creature in the eye.  It looks kind of ghoulish... he really hopes it isn't undead, undead familiar plus his current alignment would make him an easy sorting to Urgathoa's realm.

"So uh, um... uh" damnit his stutter is back.

Deep breath in, feel the words, quick mental prayer.

"The captain and first mate know there is some sort of magical creature loose, if they didn't set me on they wouldn't have had other crew looking.  My spiders found you first... I have healing if they hurt you... I don't have very good control over them but really I'm actually better than standard at that particular spell and it was the best I had for searching continuously" he is kind of babbling, he needs to organize his thoughts...

He pauses for another breath in.

The ratling will let him talk, its eerie focused gaze never leaving him.

So kind of it to patiently wait, it reminds Fernando of his likely deceased (but now free in the Maelstrom) mentor.  Yeah, just think of it that way, it is not creepy at all!

He starts again.

"The captain and first mate are really worried by a strange magical creature aboard, but, so far, they only have one definitive sighting and my report of my spiders attacking and teleportation type of spell used.  One easy way to reassure them would be if they believed you became my familiar... familiars... you know how wizards have magically augmented creatures as helpers?"

"I have seen some spellcasters in the company of animals. The purpose of this is not clear to me."

“Well for the animal it gives them a longer life, and a big boost to intelligence if they weren’t able to speak before… uh I uh, I don’t know if it would help you since you can already speak… and they share in some of the wizard’s magical powers… that is probably redundant and worse than what you already have but it might add some versatility and more options.”

He pauses for any questions.

"If I believe I am about to die, I see no reason not to tell everyone in earshot of your strange poetry and fancy spell scaffold. With that in mind, do you believe that pretending to be your familiar is the best way for me to survive this voyage?"

If he went to kill this rat-thing, he would do it with the crew the first mate trusts, and they already know he is higher circle than he pretends to be.  Still, it’s best if the rat-thing thinks it has the upper hand.

He allows a trace of anger followed by a worried contrite expression to show on his face.

“I uh, definitely want to avoid that… so I want to keep you from dying.  I think we’re a bit over a week from our next stop, maybe two if the winds don’t improve or three if they get even worse?  So assuming you don’t mind living on or under the streets of Corentyn you could possibly just hide for that long then get off the ship instead of pretending to be my familiar?  If you don’t need to take any more food, and you can stay unseen by the crew I think the first mate won’t want to bother you any more.  If you have at will dimension door or at will invisibility or similar illusion or mobility so you can stay hidden that’s probably the best plan for you actually?”

He’s rambling a bit, but he is worrying the rat-thing has some sort of lie detection or mind reading and doesn’t want to outright lie.

“If you’re more limited than that… uh, yeah my familiar plan is actually pretty good.  And also Cheliax, including Corentyn, is kind of terrible.  I bet you’d like Absalom better, you wouldn’t even need to hide yourself Absalom has so many exotic things.  Stick with me and you’ll get to Absalom.”

He’s probably overselling Absalom, but he is confident that Cheliax is bad.

”For myself, I’ve considered getting a familiar for a bit… I had a bonded object before but the extra spell is less of a priority when you’ve got more spells.”

"I can stay hidden well enough. If you did not take the food from my nest, I'll have enough to last until Corentyn. The bargain you offer does not seem worthwhile to me."

This 'familiar' idea is intriguing, but he can wait until he makes it out of Cheliax and spend some time scouting for a good deal. There's no reason to hitch his fate to the first prospect he meets.

“Oh uh, we took the food in the other nest back, but yeah, but I can get you more food, especially if it means you stay out of sight.”

And then he can use that interaction to try to learn what languages the rat-thing knows.

”I can bring more food here, tomorrow night, if that works for you?”

"Yes. It should be meat."

"When was I seen?"

It probably won’t hurt to share information, and he wants to build rapport.

“Middle of the night, four days ago.  One of the crew has some tiefling blood (although you can’t tell just looking at him) and could see you even in the pitch black, at least well enough to tell you were bigger than a normal rat.”

"Ah."

"Is there anything else?"

“Do you have something you prefer to be called?  I’ve been going by Marit lately, but my name is actually Fernando.”

He’s really gambling that the rat-thing isn’t a Fey and doesn’t have spells that get stronger knowing the target’s name, but he really wants to get this creature to trust him.

"I have been called many things, but do not have a name, if I understand the concept correctly. You may as well call me white-face."

It's one of the things his rats would call him, and most of the rest don't make much sense coming from a human.

It's late at night, he doesn’t think anyone is coming by anytime soon, so he might as well see how long he can keep the conversation going and get to his most important question.

“Your Taldane is quite understandable…  Is it your first language or do you have others you know?”

"I have magic to speak languages. It is called tongues in taldane, I believe."

He’s heard the breaking of prophecy means the gods are less able to arrange coincidences, so that must mean his god had to exert themselves to arrange this perfect opportunity.  A familiar with constant tongues could really help him practice a lot of languages!

He switches to speaking in Protean.

“That’s quite a useful ability.  We can avoid being understood by anyone listening in that way.  And it’s not that suspicious for a wizard to have learned an unusual language.  This one is my favorite.”

It’s the language he speaks his poetry in. 

He switches to protean as well, although he doesn't bother speaking slowly.

"It's not a normal language, is it? It has many ways to say things."

He doesn’t have any translation up, so it takes a moment for his brains to catch up with the words, but once it does he is sure he understood.  His reply is a bit faster and free of his usual stammering.

“Normal languages transform over time as people change the words bit by bit and learn the changed words over time.  For example, a king speaks with a lisp, his vassals and servants imitate him, and a new variation of words slowly spread through a region.  The language of Maelstrom changes more swiftly and willfully.  A group of Proteans decide on a new variation of a word, and within hours or even minutes every creature across creation with a tongues spell or similar magical speech can speak and understand that variation.  Even a lone poet (with enough intent and stubbornness) can by repeating a new word to themselves make its meaning stick to the Protean language and thereby spread throughout creation.  Of course old words can slip away just as easily.”

"Why do you compose those poems?"

“If I wanted to sound grandiose… I like the idea of my words echoing across all creation in some small way.  But the most important part to me is… I had… a group, we were all trying for the Maelstrom, you know, be Chaotic Neutral…”.

He’ll leave out the part about following a Chaotic Neutral God.

“…they’re probably dead now.  And so, I like to hope that they’re out there in Maelstrom, talking with whatever Proteans or whatever they come across, and some of my invented words might find there way to them.”

Hopefully White-face will find him passionate and loyal and not pathetic?

"...What is the Maelstrom?"

He has heard some things about the afterlives, but not a lot, and what he has heard was pretty confusing.

(Fernando's sentimentality seems in-character for a human, although it's not as if the ratling really gets it.)

“Have you heard about the afterlives?  After your body dies your soul passes on and is judged according to Law and Chaos and Good and Evil and what God you followed and then sent to one of 9 afterlives.  3 of them are really horrible and easiest to get sent to, 3 of them are pleasant and nice but get that way by only letting very particular people in so they’re hardest to get sent to.  The Maelstrom is in between, reasonable to get into and not too bad once you’re in, especially if you’d rather not deal with cities or any coherent structure.”