Never trusted octopuses, I didn't. Those crafty bastards are smart, I tells you. A little bit too smart... 

                                                                                                                                                                                                                                        — Jobin Starre, retired sailor 

Hyper-intelligent sentient Octopi with the ability to take control of a host body. Complex and curious creatures, they're obsessed with attaining new knowledge, one way or another...

Curious Mysteries 
The Cephyllith are a rare race of sentient octopi... Their origin is a mystery that many of them wish to answer, but some speculate they are the descendants of magical experiments gone wrong, which escaped to the ocean. They are intelligent, solitary, curious creatures, obsessed with novel objects and new information. In the ocean, they often reside around sunken ship-wrecks,  using wits, dexterity, and tools to explore every nook and cranny, determinedly breaking into chests or drawers, collecting treasure, trinkets, and, usually... more tools.
Disguised Denizens of the Deep
Physically they appear as normal octopi, except slightly bigger, and their skin can change to whatever color or pattern they choose. They stand at about 2 - 2.5 ft high and weigh about 6-8 pounds. The Cephyllith share the octopus' ability to drastically change its body shape and can fit through any space of 1.(Strengthx4) inch in diameter or more (the size of their beak). Alone, Cephilliths are somewhat vulnerable: relatively weak (although surprisingly strong for their size) dependent on water, slow-moving on land, and unable to communicate verbally. They have no natural defenses other than their intelligence and ability to change their skin to mimic the most complex of surroundings. 
Parasites of the Mind
What makes the Cephyllith powerful is their ability to take a beast or humanoid as a host: by inserting a thin, sharp spine into the base of their brain stem.  Once a host is taken, they have complete control of their physical movement, speech, and actions, as well as full access to anything that creature knows (languages, abilities, skills, memories etc.) Their camouflage and flexibility allows them to imitate the face and head of their host victims, or any other face they have seen, and will only be discovered by the very perceptive, or a close investigation.

Cephylliths, although leading longer lives than normal Octopus, still only live to around 20 -30 years. This encourages their obsessive sense of curiosity and demand for new knowledge and experiences and perhaps explains why most of them are willing to ignore the plights of their hosts. Very few creatures would be willing to sacrifice their freedom to a Cephyllith, and their constant resistance to the domination is the Cephyllith's greatest weakness. 
However, being able to steal the experience, knowledge and abilities of whichever creature they control, they can be fearsome opponents. Thankfully, most Cephylliths are content as ocean-bound hunters: attaching themselves to larger predators such as sharks and giant squids, or an opportunistic Cephyllith might 'rescue' a drowning sailor from their deaths, only to take over their minds - and perhaps their ships. However, the more intelligent and ambitious Cephyllith's have been known to devise even more complex schemes to seek ever more intelligent and powerful prey. 

Solitary Strangers
Cephyllith's tend towards neutral and chaotic alignments, as they are solitary creatures with little investment in the laws and sensibilities of communal land-dwelling creatures. It is not to say they do not have their own peculiar moral codes, simply that they have a very relativist view of morality. Like detached, alien anthropologists, they are more interested in studying the beliefs and actions of other creatures than in obeying them. Their obsession with the novel and their curiosity may motivate them to play along with the rules. However, like any anthropologist, they are strangers in a strange land and find it hard to understand the strange ethics, customs, and cultures of land-based creatures. The best intentioned of them might become brave adventurers, taking control of malicious minds and putting them to good purpose, or they may even find a willing host and a powerful symbiosis. The most malicious of Cephylliths can become cruel and unusual experimenters on the psychological limits of others, pitting people against each other in vicious and violent ways.

Names

Cephyllith chose their own names. Commonly, it is an unpronounceable onomatopoeia of their favourite underwater noise, the Pluhnk of a stone hitting the surface, of the Blulp of a popping bubble. More intelligent Cephyllith will choose from any range of names they have heard in their lives, often changing as often as every six months, when a new favourite name takes their fancy. 

Cephyllith Personalities

Cephylliths are obsessively curious creatures and can be compulsive in their ambitiousness. For many creatures, information is a means to get power... but for the Cephyllith, power is a means to get more information. They usually have at least one kind of power or information they are particularly obsessed with attaining. However, this can change.

 Cephyllith Obsessions

Cephyllith (Mimic Octopus) Traits

+2 Intelligence, +2 Dexterity , -2 Strength, -1 Charisma, -1 Wisdom. Underwater Masters. Ink Cloud. Uncanny Camouflage. Grappling Attack. Take Host. Immunity to Grapple and Prone.

Ability Score Changes

Intelligence +2
Dexterity +2
Strength -2
Wisdom - 1
Charisma -1

Language

Cephyllith's speak Aquan, and any languages they have learned from their host bodies.

Limited Armoury

Cephylliths alone cannot themselves wear any armor, and they get disadvantage on attacks with any large weapon or bow. They may be proficient in any armor or weapon, but can only use them with a host body. Alone they may still use rings.

Walking Speed

Without a host, Cephyllith's have a walking speed of 15.

Size

Cephyllith's are 2-2.5ft tall, and weigh around 6-8lbs. Their body size is small.

Age

Cephylliths live longer than normal octopi, but still live short lives of 20-30 years. This perhaps encourages their reckless curiosity and ambition.

Underwater Masters

Cephyllith's get advantage on Strength and Dexterity saves/ability checks while underwater, and have a speed of 40 feet whilst swimming.

Water Reliance

While out of the water, and without a host, the Cephyllith can hold its breath for 30 minutes. 

With a host, they can breathe normally, and tap into the body for water. They must drink an extra gallon of water per day on top of the normal food and water requirements of their host. 

Ink Cloud

A 10-foot-radius cloud of ink extends in front of the Cephyllith. Any creature caught in the area must make a Dexterity check (DC10 + 1 per level of the Cephillith) for no effect, or be blinded for 1 round per level of the Cephyllith. After releasing the ink, it can use the Dash action as a bonus action. If underwater, the area is heavily obscured for 1 minute, although a significant current can disperse the ink. 
After using this, the Cephyllith cannot use it again until it has taken a short rest.

Tentacle Strike

You are proficient with your unarmed strikes, which deal 1d4 bludgeoning damage on a hit. One a successful hit, you can attempt to grapple the target.

You gain an extra attack per turn at levels 3, 9, and 18. (This does not apply to the host creature).

Natural Grapplers

The Cephyllith's tentacles give it advantage on all grappling attempts. Escape is Athletics (Strength) or Acrobatics (Dexterity) against the Cephyllith. Until this grapple ends, the Cephyllith can't use its tentacles on another target. If the creature fails its save, the Cephyllith may use a bonus action to attempt to 'Take Host'.

It can still use its Ink Cloud attack while grappling. It can attempt to grapple creatures of any size, but does so at -2 for every size above medium. 

Take Host

When the Cephyllith has successfully grappled a humanoid or a beast, it may use a bonus action to attempt turn that creature into a host, by inserting a long thin spine into the base of their brain-stem. The target must succeed on an Intelligence save vs the Cephyllith, or be completely dominated. They can still see, hear and feel everything that happens, but lose all control over their physical actions, speech etc. The Cephyllith retains its Intelligence, Wisdom, and Charisma scores while taking the Strength, Dexterity, and Constitution of the host. The two creatures retain separate hit dice and can be injured/killed independently of one another. A Cephyllith with a host can rest normally.

Whenever the host creature or the Cephyllith takes damage of half their current hit points or more, the domination is briefly weakened. The Cephyllith must save on a constitution save or drop their camouflage for 1d20 seconds (minus their level, to a minimum of one), and the host may make another Intelligence save vs the Cephyllith (with advantage) to resist the domination. Any critical hit against the Cephyllith or the host body will have the same effect.

Attack damage is otherwise conferred to the host body, unless the attacker specifies attacking the head, at which point damage is halved between the Cephyllith and the host body. Any healing must be applied to the host and the Cephyllith individually.

If successful in resisting the domination, the host must still save against the Cephyllith's grapple (Strength or Dexterity). 

If a creature is asleep, stunned, or otherwise incapacitated, they get a disadvantage on their Intelligence saves against being taken as a host. 

The Cephyllith may use an action to willingly disengage from a host body. If they do so, they may use a toxin to stun the creature for 1d4 rounds (plus their current level, divided by two, rounded down.) The host can make a Constitution save against the Cephyllith to half this duration. The DM makes this roll for them, and the Cephyllith never knows how long their host will be stunned for.

While in control of the host, the Cephyllith has full access to their knowledge, memories, language and abilities. During this time, they may allocate any experience earned towards one of the classes of their host creature. To achieve level one in that new class, they must earn the experience needed to achieve level 2. To achieve level three, you must achieve the experience needed for level 4, and so on. This only applies when in control of a host that has the class you wish to learn.

Outside of this, normal multiclassing rules apply. DM discretion as to whether the player has used their hosts skills sufficiently to permanently learn them,

Example: A level 3 Rogue Cephyllith takes a level 2 Wizard as a host. During that time, they earn enough experience to achieve level two as a wizard, and permanently achieve level one as a Wizard. After this, the Wizard host dies, and now, to get to level 2 as a Wizard, the normal Multiclassing rules apply, and the Cephyllith must earn enough to get to level 5 (Rogue level 3 + Wizard level 1 = 4 levels total)

 

Uncanny Camouflage

The Cephyllith is able to camouflage its body to almost perfectly imitate its surroundings. Even complex, shifting backgrounds, or taking on the appearance of different faces on a host body. The face must belong to the host body, or be one that the Cephyllith has seen before. They cannot change the appearance of the host's torso or limbs - only the face and head. 

Cephylliths gain advantage on all Stealth checks. This only applies to the Cephyllith alone, not to the body or actions of the host body.

Quick and Slippery

While not in control of a host, the Cephyllith has an advantage on all rolls against being Grappled or Knocked Prone.

Aloof Loners

Cephylliths tend towards chaotic and neutral alignments, as they are solitary creatures with little investment in the laws of landbased creatures.

Hybrid Nature

While taking a host creature, Cephylliths have two creature types: monstrosity and the creature type(s) of their host. They can be affected by a game effect if it works on either of your creature types.

Im not sure how to go about the creatures carrying capacity. Should they be able to use backpacks? Is a small creature with -2 to strength limiting enough?

I have a few questions.

What is state do you want a character to be in more of the time? With a host or without a host?

What is their walking speed?

Why did you add Water Reliant?

Why did you make this race?

1) That depends on the player, and how it would be used. A willing host would be permanent.  Another cephyllith might enjoy moving from creature to creature. There are a lot of options for adventuring when you can move creature body as you go.

Examples. 

A level 2 rogue Cephyllith has a level 1 fighter with a terrible dex, for a host. They come across a locked door and no one in the party can pick it (the hosts dex is too low) The Cephyllith stuns his host and picks the lock as quickly as possible with his/her higher dex, then returns to the body. (Hopefully before the host wakes up and starts fighting back.)

Another Cephyllith loses his host (who he'd managed to keep for a few months) in a fight. For the rest of the adventure, he is forced to go in Octopus form, making things difficult. However, it means they can send the Cephyllith ahead as a very good scout, and to try taking hosts from their enemies amongst the dungeon. First, maybe they get an Orc host, who lasts three battles before damage makes it resist the domination in their fourth battle, and they are forced to kill it. In their next battle they almost kill an Ogre. Instead, they give the body o the Cephyllith, and then heal it. Later, maybe they find someone with knowledge and power? They try interrogation but it doesn't work. Maybe the Cephyllith can take over their minds and learn key information about the quest?

There's honestly SO many ways this could be played. That was kinda the point.

2) Oops I forgot to write that. Without a host,  walking speed is 20. Could be 15? Maybe? Needs playtesting. They aren't very fast out of water. What do you think? 

3) Because its an octopus? (and to give an extra setback to a powerful creature, and to give aother way they can be spotted/detected.) Does that make sense?

4) Main reason is for a module im creating. Because its different to all the other races, there hasnt been a body snatching race, as far as Im aware. I wanted something really different, that wasn't just another boring humanoid race. This race allows interesting mashups of abilitiies and multiclassing in a new (but not overpowered) way. I wanted a creature that had the ability to be incredibly powerful, but also very weak and vulnerable, and would be interesting and maybe a bit more complicated to play.

Also, 4) I was watching a video on the intelligence and abilities of the Mimic Octopus, and thought of Mimics and Illythids... and then Cephylliths were born. 

ps. Pratchett is my favourite.

The body-snatching is a problem. It can be a very powerful ability and can turn every monster into a potential host. As you have no limits on what types of creatures you can use this ability on a ancient black dragon or a fire giant or a beholder.

A speed of 15 is reasonable compared to the general power.

I think Water Reliant is, well, not prohibiting in the way one might want it to be. I just see it as another thing to juggle as I'm taking a short rest or casting Find Familiar. As a DM, I would tell my player to disregard it.

Underwater Masters is worded poorly. I don't see why a Cephyllith gets advantage on Arcana checks just because it's underwater. I can understand advantage on Dexterity and Strength checks.

I notice that Grappling Attack is worded like a monster attack, is this intentional?

Immunity to Grapple and Prone states is a mouthful, how about Slippery? I can also see an octopus going prone.

A Cephyllith is to put it lightly, not a humanoid. Give it the Hybrid Nature trait, found in the UA Centaur and Minotaur.

1) Thats a really good point about doing 'any creature'. My thought was that the intelligence saves against domination would make something like dragons next to impossible. I first thought about limiting it to humanoid creatures. But that makes no sense for the evolutionary backstory of the Cephylliths (hunting in the ocean). So perhaps it could be a size limitation? Or creatures with a lower intelligence than the Cephyllith? Or creatures with no natural armour? (I dont imagine the Cephyllith's 'brain spine' penetrating dragon scales)? Also, dont Dragons have the legendary ability to automatically succeed a saving throw (or two) every round? Wouldn't that basically make it impossible to dominate one?

My thought was that for the majority of very powerful creatures, their own natural resistances and abilities would make 'hosting' such a creature VERY VERY hard. Still possible, but highly unlikely.

2) Will make walking speed 15.

2) Well explained. I feel there does need to be some limitation on their time spent out of water. My other thought  was just that the Cephyllith uses the water of its host, and just requires one or two extra gallons of water per day (not so complicated).

I suppose the half and hour of holding breath out of water is limiting enough? Or they could simply start taking damage after they've been half an hour out of the water without a host? What do you think?

3) Yeah, I wondered about that. Will correct to Strength and Dexterity checks. 
4) Probably because I copied it across from the Octopus creature attack. Should it be more like this?

"Tentacles: You are proficient with your unarmed strikes, and may make two attacks per round. One a successful hit, the host creature must save on a Strength or Dexterity check against you or be Grappled." 

5) Thanks, 'Slippery' is much better. And yeah, perhaps immunity to Grapple is better. But I'm not sure I can envision a 'prone' octopus, as they dont have a 'back' as such. But they could be put upside down, and would still take a moment to right itself, so I suppose you are right. 

6) Hybrid Nature is perfect.

Thanks heaps for the questions, this is exactly the critique I needed. This is my first module, so thanks for lending your experience. Have made some edits.

1) Limit it to beasts and humanoids.

2) The hold breath trait and the gallon rule is perfect. There are already rules for suffocation.

3) You should be able to edit the post on desktop.

4) Try

You are proficient with your unarmed strikes, which deal 1d4 bludgeoning damage on a hit. One a successful hit, you can attempt to grapple the target.

Two attacks is far too much for a first level character, even if it's an unarmed attack.

Thanks!

Since they have eight arms, I feel like they should maybe get more attacks as they go up levels then? (This would only apply to when they didn't have a host). Something like 2 attacks at level 5, 3 at level ten, 4 at level 20?

Or is that just overcomplicating an already complicated creature? Should I just leave the number of attacks up to whatever class the Cephyllith in question chooses to be?

Damn, I just realised I forgot one of the most important parts: the ability to permanently learn from their hosts. 

I'm not sure how to word it, but it makes sense that while controlling a creature, the Cephyllith can learn from them permanently, i.e. learn their languages, remember parts of their knowledge, and go up levels in their Hosts classes a little faster than the normal multiclassing rules allow. This is (vaguely) how I imagine it working.

While in control of the host, the Cephyllith has full access to their knowledge, memories, language and abilities. During this time, they may allocate any experience earned towards one of the classes of their host creature. To achieve level one in that new class, they must earn the experience needed to achieve level 2. To achieve level three, you must achieve the experience needed for level 4, and so on. This only applies when in control of a host that has the class you wish to learn. Outside of this, normal multiclassing rules apply. DM discretion as to whether the player has used their hosts' skills sufficiently to learn them permanently.

Example: A level 3 Rogue Cephyllith takes a level 2 Wizard as a host. During that time, they earn enough experience to achieve level 2 as a wizard, and permanently achieve level 1 as a Wizard. After this, the Wizard host dies, and now, the normal Multiclassing rules apply, and to gain another level in any class, the Cephyllith must earn enough to get to level 5 (Rogue level 3 + Wizard level 1 = 4 levels total). If they want to level up faster in any class, they will have to find a host of that class.

That's really broken. It's going to cause strife for both DMs and players. I suggest perhaps giving an extra language and skill but at this point any additional features is going to be too much. Extra attacks should be governed by the class. 

Yeah, I was kinda pressed between knowing it was probably overpowered and complicated, and what makes sense for a creature that takes over minds with the intent of keeping their knowledge. I feel like some ability to permanently learn the language/skills/abilities of a long term host is realistic.

What about something more simple like - 'Whenever you earn a full level's worth of experience while in control of a single host,  you may choose one Language or Proficiencies of your host creature to learn permanently."

Or the character could take time out of their day to practice a certain skill of their host? Like 'Once the Cephyllith has spent 50 hours (minus their intelligence) in control of a host, practising that skill, they can learn a Language or Proficiency.

I see what you are saying, but just choosing an extra language or skill at the beginning - 'from a past host' as it were - doesn't really reflect the creature's ability, and a level one Cephyllith may never have an intelligent host before.   Or did you mean they get to choose and extra language or skill with each level they go up?

Perhaps you can give the following trait.

Stolen Knowledge. Starting at 3rd and 5th level you learn one language of your choice, and you are proficient with one skill or tool of your choice.

Cool, thanks so much. How would that mechanism be created in the Race editor? I dont see a way you can set the modifier to happen at certain levels.

Also, as an aside, how detailed does such a homebrew thing need to be to get put up publically? Do I need to create variations of Dominate person spell that the 'Take Host'  attack emulates? Or can I just write the description of what it does?

As I took a look, it appears that mechanic wouldn't work. I suggest you explain what you're trying to do over in the Feedback forum. Until then, the race won't break if you give stolen knowledge at 1st level.

If the ability is not a spell, don't attach spells. Yes, the description will work fine. Please don't forget snippets!

Thank you so much, your expertise is appreciated.  Im looking forward to playtesting this weirdness. My players are in for a strange time.

Would it be unreasonable to make their attack modifier their Dexterity instead on their Strength?

Nope! Unusual, yes, unreasonable, no.