“and Mofu says mofu mofu”

-Daikael, 2023

Agony and pain and my body’s to blame. 

This becomes more than obvious upon achieving one of the rarest isekais known to isekai-kind - isekai-ing into a body part. You will not even get control over a full body - just a part of it, and this comes with many, many complications.

However, not all is doom and gloom upon isekai-ing into a body part. There are some benefits to it, if you can pull it off. Do remember the key rule of isekai-ing into a body part - preserve the host to preserve oneself. If your host dies, so will you. In effect, you have become akin to a symbiotic parasite. You help your owner grow, they help you grow. Thus, it is important to keep this in mind as you grow in this extremely rare and vivid type of isekai. 

Host

There are quite a few benefits to having a host - as a body part, you are practically required to have one. A host can supply you the nutrients you need to stay alive, can keep you protected when times get tough, and make the terrible decisions when push comes to shove. They can also move you around, enabling you to reach for new enemies and gain new results from them. They can also choose to enhance you, pushing you to greater heights than if you were alone, and cooperate with you if you can establish your sapience. 

However, having a host has its drawbacks. For transportation, you are dependent on your host, meaning if your host settles down, your travels become limited to your range of motion and nothing more, depriving you of possibilities to grow even further. Additionally, the host can choose to neglect you in favor of something or someone else, meaning your development could fairly easily fall behind (think a magic organ in a person who specializes entirely in muscles). Furthermore, you could easily be abandoned by your host, since your host is likely to prioritize their survival over yours - since you are just a parasitic-like existence. They will likely view you as no more than just part of them, especially those with more entitlement and arrogant attitudes such as those in nobility. And finally, if your host perishes, you lose your source of nutrients, meaning you are likely to perish soon after. 

Location

Location, location, location. It is a popular saying in real estate, no matter, which universe you go to, and it applies even more so to you as a body part of another entity. Where your location is determines a lot from your normal abilities to your durability to your usage, so location is key. 

If you are located on the outside of the body, such as the skin, horns, tails, nails, or occasionally even hair, then prepare for longer periods of waiting and inaction followed by shorter periods of hectic action. These organs are only used upon being called upon, with tails and skin typically being defended by equipped armor, nails and horns protected by gauntlets, and hair protected by helmets. This means you must be ready for sudden jolts, and your development in the future should reflect the specific functions which will benefit you and your host - think hyper-regenerative skin or tougher nails. 

Additionally, as an external body part, you require far more conscious maintenance compared to other body parts, making it easier to fall into disrepair as your host uses you without rest. Thus, you do have to worry more about regeneration and durability, oftentimes putting that above all other abilities to stay alive.

If you are located on the inside of the body, which is even rarer compared to isekai-ing into an outside organ, then woe is you. Internal organs are kept working at all times, such as the heart, liver, lungs, and muscles. (Yes, even muscles, since they have to constantly work to stretch and contract). These organs also serve a far wider variety of different functions, meaning you may have access to an even larger variety of powers compared to external organs, in exchange for the constant work. The liver, for example, filters the blood of many, many different compounds, along with storing certain proteins and bile for future use. Thus, you must be ready to manage many, many functions at once, and at any time. With great power comes great responsibility. Do note that due to being an internal organ, you are far likelier to be extremely squishy, meaning your primary goal should be to help your host either grow as durable as possible or as quickly as possible, to defend against attacks. 

Furthermore, as an internal body part, you require far less conscious maintenance compared to other organs, due to your protection by the external body parts. Thus, you have less to worry about when it comes to regeneration and the like, making it more convenient to focus on other abilities such as protein production and enzyme analysis. 

Raw Growth

In terms of raw growth, this mostly depends on which body part you have become. If your body parts contain lots of muscles, such as a tail, then raw growth is obvious. The stronger your muscles become, the better, and the more durable they become, the longer they will last. However, if your body parts contain less obvious organs, such as hormone-producing organs, then raw growth is far more difficult. The more proteins you can produce, the more effects you will have on the rest of the body, such as the bloodstream. 

However, some consistencies are useful for you to know when isekai-ing into a body part, specifically regarding size and efficiency. 

Larger does not always mean better. The larger your body part is, the more resources will be needed to sustain you, and the more unwieldy it can become. Thus, you need to establish a delicate balance between raw power and raw size, since raw growth can lead to both in almost every case. Particularly so for internal body parts - most internal organs are snug-fitting, meaning growing larger could do more harm than good. 

However, efficiency is useful for virtually any body part, since it reduces resource consumption, and thus indirectly makes you and your host more powerful. If you can, push efficiency as high as possible since it will mostly lead to only benefits down the line. However, efficiency can lead to a decrease in raw growth, as resources that could have been spent growing the strengths of cells are spent keeping it in its ideal state as long as possible. 

Raw power is also something you must be able to keep consistent when growing, else wise it could cause trouble. A lack of it means your host will not be able to utilize you to the fullest, but too much of it can lead to troubles in daily life. Either way, your host will not be as likely to use you, meaning fewer resources sent your way and less attention to your needs. Additionally, certain hosts can be paranoid when it comes to having other sapient beings within them, meaning you must keep your raw growth as natural-seeming as possible. (And thus one of the most ideal times for growth is when your host is asleep). 

Communication

This is one of the most important, yet riskiest, parts of isekai-ing into a body part. When it comes to communication, the better you can effectively communicate your desires and the information you are aware of, the more you can assist your host in growing themself and yourself. Communication enables mutually beneficial cooperation between you and your host.

However, communication can also cause the worst of the worst to happen - your host rejects you for who you are, which is a part of them. Many hosts dislike having sapient parts of their body act independently of them, feeling terror and disgust as their own bodies seemingly “rebel” against them. This can lead to your quick, untimely demise, as they cut you off from your sole source of nutrients and transportation - such as by cutting their tail off (you) and regenerating a new one. When things go south, damage to your isekai can occur quite quickly, and thus it is necessary to take several precautions and steps when attempting communication. 

1. Judge your host. If your host is amicable to those around them, it just might be possible for them to understand that you have suddenly come to life in their body. If you have attached to a newborn, just the newborn and the mother - the mother, in particular, might become overprotective and chop you off to save their offspring. If there is no parental figure, you can judge the newborn as vulnerable to your persuasion. 
2. Take it slow. Due to the wary nature of many host protagonists, oftentimes it is not the best choice to reveal your sapience immediately. In many fantasy worlds, sentience in objects is not abnormal, and thus you could attempt to portray yourself similarly to those. Start with performing actions that the host did not call for, but just enough that they can notice it. And only just enough, when the time is opportune. (Ideally, they should be alone and open-minded to learning more about things, although this does vary from host to host). This is the step that most isekai-ers who isekai into body parts fail at, having rushed through it and frightened their hosts, leading to disastrous consequences. This could be anything from twitching their tail (you) when they are sitting down to the horns getting more irritated than normal (you sending more signals to their mind as their horns). 
3. Gradually open up as they continue observing (and occasionally experimenting on you). Start making slightly larger moves, such as a tail swishing even when they have never told it to repeatedly swish in the air. This step should only be attempted once they have acknowledged that their body part is abnormal, and thus they may also be experimenting. If they experiment on you, look for ideal openings to simulate pseudo-conversations, such as becoming more reactive when your needs are met. (Think of a horn likes being covered in horn growth formula, then it will rapidly send pleasure signals when drenched in it instead of the typical irritation.)
4. Attempt pseudo-conversation. This could be as simple as making basic movements to simulate yes or no, or going as far as controlling parts of their body to make movements for you. This is the most dangerous step - take this step too far too quickly, or say the wrong message, and you could get exorcised since demonic possessions in many worlds also take this route. NEVER attempt this step if they are not completely comfortable with you in step 3. (Do note that you may need to come up with an excuse as for why you are sapient - some common excuses are that the gods blessed that part of the body, that they’ve cared for this body part so much you appeared, or that they have a rare mutation that allows them to have a parallel mind which can control that particular body part). 

Most isekai-ers into body parts will rarely make it past step 2, as hosts become wary of their newfound extra sentience in their body. Thus, isekai-ing into a body part is not only rare, but it is also highly unpopular, making finding experienced veterans to interview and learn from about this type of isekai difficult. 

Ultimately, as a body part, you work in conjunction with your host, whether for better or for worst. Keep your host happy, and your host will grow you as well. And remember to keep them updated on your growth - they will still be using you, after all, since you are a part of them. Body dysphoria is no joke, especially when it can be fatal. 

AN: This was so painful to write since I had only one example of an isekai to work off of.