This spoiler, written by Eva Myers, deals with the Doppelganger race and their #youpoly command. It mostly assumes that you are a Doppelganger; Flame Mages, Ice Mages and Lycanthropes can also use #youpoly, but can only polymorph into specific monsters and are not covered in detail here. Some of the information may be useful for non-Doppelgangers who are polymorphing with the aid of wands, spells, potions or rings, even in vanilla Nethack, but this is not guaranteed. If you have any comments, additions or corrections, please feel free to email me.
There are a few conditions which must be satisfied before you can #youpoly. Obviously, you must not be wearing an amulet of unchanging. If you are in your normal doppelganger form but have less than 20 energy, you won't be able to polymorph. (Flame Mages, Ice Mages and Lycanthropes have different requirements. For example, Flame Mages can polymorph into a red dragon for free if they are wearing red dragon scale mail.)
If you use the #youpoly command while polymorphed, you will return to doppelganger form. This does not consume any energy or nutrition. Flame Mages and Ice Mages can also do this, but Lycanthropes need energy to rehumanise.
Otherwise, you will be prompted for the type of monster you wish to polymorph into. The prompt is ``Become what kind of monster?'' if you have polymorph control - Doppelgangers gain intrinsic polymorph control at level 9 - and ``Attempt to become what kind of monster?'' if you do not. It's useful to try #youpolying while wearing an unidentified noncursed ring to see whether it's polymorph control.
You will never be permitted to polymorph into any of the following monsters, and if you select any of them (or something that doesn't exist), you'll have another chance to choose a monster to polymorph into, up to five tries.
Undead Slayers' gods will not allow them to change into an undead monster. If they have polymorph control, the attempt was clearly deliberate and they take an alignment penalty of -5 and abuse their wisdom. If it was a successful attempt by an inexperienced Doppelganger, there's no penalty, but not much point either since it won't work.
Even if the monster you've chosen is OK to polymorph into, it may be too powerful for you to take its form yet. If you have polymorph control, or have eaten the monster, there is no problem - you have a 1/20 chance of feeling like a new doppelganger, and otherwise will become the monster you want to become. Otherwise, the probability that you will take the form you want is dependent on the monster's level and your own. If you fail, you will turn into a random monster (or feel like a new doppelganger) instead. The probability of failure is
(Monster's level - your level) / 5.
When you successfully use the #youpoly command as a Doppelganger, you use 20 energy points. If you actually changed form, as opposed to feeling like a new doppelganger, you use an additional 5 energy points per level of your new form. If you don't have enough energy for this, the excess is made up with nutrition points.
If you are a Doppelganger or Lycanthrope, you will drop your body armour, cloak and shirt rather than breaking out of them, unless they are cursed.
Polymorphing into a strong monster sets your strength to 18/** while you are polymorphed. Polymorphing into a stoning-resistant monster cures stoning, polymorphing into a sickness-resistant monster cures sickness, and polymorphing into a fire elemental, fire vortex or green slime cures sliming.
When you are polymorphed, your physical stats are temporary. You cannot exercise or abuse stats other than wisdom while polymorphed, and any changes to your stats will be lost when you return to doppelganger form. So while polymorphing may make you more powerful for a while, it also prevents you from improving your stats. On the flip side, you can eat poisonous corpses while polymorphed and not suffer permanent damage.
[2 possible bugs here. Changes to your Int and Wis are also lost when you return, and if the exercise counter is checked leading to a message like ``You feel strong! You must have been exercising'' while you're polymorphed, only your polymorphed stats change.]
You will remain in your polymorphed form for 500-999 turns, multiplied by your level divided by your new form's level, and multiplied by 2 if you are a Doppelganger and have changed into a monster you've eaten. You can prolong this time indefinitely by wearing an amulet of unchanging.
If you run out of hit points while polymorphed, you will change back to your normal form (unless you have the unchanging intrinsic, of course). However, if you die in any other way (e.g. turning to stone or being hit by a death ray), it will be permanent.
If you revert to your normal form by the #youpoly command or because your polymorph time ran out, you will come back with at least as many hit points as you had before you polymorphed (barring a few obscure actions like eating cursed royal jelly which knock down your real HP even though you are polymorphed). The way natural healing seems to work while polymorphed is that if your polymorphed form is at below max HP, it will heal, and if it is at max HP, your real doppelganger HP will heal, all at the normal rate as affected by regeneration and healthstones.
However, if you revert because your polymorphed form died, you lose half your real HP. Non-Doppelgangers may also lose half their real HP if they fail a constitution check when their polymorph time runs out.
If your polymorphed form was killed by a monster or explosion, you may also have a number of HP equal to your polymorphed form's level deducted from both your current and maximum HP. This penalty will be extracted every time if you have polymorph control and 1/3 of the time if you do not.
If you are a Doppelganger, you will not experience all the typical effects of polymorphing into a new version of yourself. For example, your level and hit points will not change, because if they did, then repeatedly polymorphing into a human would be abusive.
Here's what will happen. There is a 1/10 chance that you will change sex. All your maximum stats, except for intelligence and wisdom, will change by a random amount ranging from -2 to +2, and your current stats will be adjusted in proportion to your max stats. E.g. if your strength was 15 (max 16), and your max strength went up to 18, your new strength would be 18*15/16. You will no longer be hungry (your nutrition points will be set to a random value between 500 and 999). Sickness and stoning will be cured. Sliming will not be cured, but the sliming process will be restarted from the beginning.
Extremely low hit points and energy will be cured. After the polymorph, your HP and max HP will have a minimum value of 10, and your energy and max energy will have a minimum value equal to your experience level.
Polymorphing into a new version of yourself is permanent, and stats which go down as a result cannot be restored with a unicorn horn, because your max stats changed too.
If I plan on being polymorphed for a long time, I prefer humanoid monsters who can wear all armour and wield weapons and have at least as much carrying capacity as I have in my normal form. (Carrying capacity depends on the weight of your polymorphed form.) Once I have polymorph control, I like master mind flayers because their brain-sucking attacks do so much damage. Ordinary mind flayers are level 9, and therefore are a possible alternative for Doppelgangers who haven't quite got polymorph control yet.
The various types of orc are reasonably good - they all have strength 18/** - and hill orcs are only level 2 so you're virtually guaranteed to be able to polymorph into one.
There are various monsters you might want to polymorph into for a short time to do something special.