RULES FOR ROLEPLAYING TRANSFORMING ROBOTS FROM SPACE IN A GENERIC OLD SCHOOL FANTASY GAME
This is explicitly and specifically about playing transforming robots that have a humanoid or animaloid primary mode and at least one alternative mode in the context of the swords and sorcery, elfs and halflings, noun-crawling fantasy roleplaying games of the OSR.
You play a robot from a distant and ancient species or group of related species, if species can even be the correct term for a technological life form that may or may not use sexual reproduction. At some temporally distance time, 65 million years or more, but really any long ass time ago works, you and/or your group of robots crashed on the planet and subsequently took a eon long nap. You may or may not be the only one awake, and may or may not have a rival faction out for your cogs and electrolytic fluids. All of that is for the Referee and you to decide. But in any case, your access to energy and advanced facilities is limited.
Lost Transforming Robot
Ability Score Requirement: None
Hit Die: d10 per level.
Attack Bonus: As a fighter.
Saving Throws: As a fighter.
Experience to Level: As an elf.
Armor: Special see below.
Special Abilities: Base AC as chainmail (5 or 15); 1d4 unarmed strike; elf chances to detect secret doors and surprise checks.
All robots start out at roughly human size, and due lack of mass displacement, their initial alt-mode is the same size, therefore not being hugely suitable for carting around more than 2 passenger (more like 1).
You roll your ability scores the same way your rules want you to.
Roll or pick for alt-mode:
1. A giant head. You are actually a Master, and as such your spark/soul/consciousness cannot be housed in a larger body, or undergo mass displacement, but you can co-opt a larger mechanical body by using your alt-mode, after severing its head. You can also eventually construct a headless body that you can control while in your head alt-mode. This can also have an alt-mode, and this can also be a head. It can be heads for increasingly large robots. It can be heads all the way up.
2. A giant weapon. You might be a different type of Master or you might be a normal robot. While you cannot hijack another robot by jacking into its neck, you can be wielded as a weapon by such robots, or be used mounted on vehicles. If you attain mass-displacement you can apply it to your alt-mode to make it a weapon for human sized creatures.
3. A wheeled vehicle. The number of wheels, or whether they are tracks, is up to you.
4. A winged vehicle. You are either a rotary-wing aircraft or a fixed-wing aircraft. Rotary-wing craft can hover but are slower than fixed-wing.
5. A watercraft. You know, a boat, like the size of a canoe.
6. A piece of equipment. Like a telescope or smelter.
7. An organic creature. Yes your alt-mode is actually a person, or a human-sized animal. You are what is known as a Pretender. While in your alt-mode are treated as a normal creature.
8. An animal. You are some kind of animal, but it is obvious that you are a machine. Yes you can chose your primary mode and alt-mode to be animals, and you can make your animal mode your primary mode with your alt-mode being your humanoid mode.
Transforming takes a full round. You're made of metal; thus your base AC is that of chainmail, and your unarmed strikes do 1d4 damage. You do not start off with integrated weapons. Getting armor made for your primary mode costs 3-5 times the base cost, and it will get irreparably destroyed when you transform.
You don't eat normal food, do not need to breathe, and cannot be poisoned. You do need highly energy dense substances to sustain yourself. As radioisotopes and other exotic substances will be hard if not impossible to find in the standard fantasy setting, you're probably fucked. Work with your Referee on how to sustain yourself. Suggestions are, spending a full-day every week collecting the life-giving rays of the sun, consuming your mass in coal or charcoal or lantern oil daily, eating the heart of a dragon, implanting the heart of a dragon within yourself, drinking the blood of an angel, demon, or other extra-normal creature, eating golems and other animated objects. All of this is ignored if you're a Pretender, in which case you must eat twice the food a normal person does daily.
All of those wonderful things from your favorite transforming robot shows and comics? You get none of that starting out, other than transforming into something cool and being a fucking robot. Those cool things you don't get initially include things like: flying outside of alt-mode, integrated weapons, energy swords, combining with other robots, turning into a city, changing size when you transform etc. Other than your size, these are not tied to your level. All that leveling does is give you more of what you get for leveling in your system.
In order to get those cool other things, you need to find raw materials, a computer and repair/manufacturing facility, and/or the components that do what you want, and in the case of mass displacement, a total rebuild, or gaining additional alt-modes, more hit dice.
You can add a level of mass displacement every 2 hit dice. You can add an additional alt-mode every 3 levels. You can increase your primary mode's size one level every 4 hit dice, which also effects the size of your alt-modes unless you have mass displacement for each one. Instead of improve your base size, you can get a build an extra headless body per a master or load-bearer armor, which is effectively increasing your primary mode's size while still leaving you free to roam in human-sized areas. Each of these improvements cost an amount of silver/gold/credits equal to the experience points needed to the second highest level earned. This might seem expensive, but you are effectively buying built-in magic items with the treasure you used to level.
Just because you are from a highly technologically advanced species does not mean you are skilled technician. You cannot "hack" computers, perform "surgery" on yourself or other mechanical things, or other things that rightfully belong under the purview of professionals. If it's the kind of thing you, the player, would consult a professional for it's not something your space robot can innately do. They can however, do a lot of things that a member of a technological species could reasonably be expected to: operate a personal conveyance, operate personal computing devices, use weapons, perform first aid etc.
Josie X's Kalak-Nur: Droid Race/Class
And Machine-Bonded Class for Old School RPGs
Allandaros of Legacy of Bieth's Mega Robot (Megaman)
James M. Spahn's Novomachina from the White Star Companion