Prostitution

In Elona, it is possible for either two NPCs, or the player character and an NPC to have sex with one another, regardless of gender (or species, relative sizes of the participants, whether or not they are undead, or even whether or not your character is even an organic life-form with appropriate "equipment" for sex).

Denizens of Tyris are apparently not terribly shy about sex, and will happily engage in sexual activity even out in the streets during any hour of day, even in "respectable" cities. (They also apparently turn a blind eye towards freakish mutant adventurers who keep pet little girls on a leash and will happily buy little girl egg omelets, so this should probably not come as much surprise at this point...)

Player Characters Selling Themselves

For those players who don't see anything wrong with sleeping their way to the top, prostitution can offer a source of income and, oddly enough, popularity with the NPCs that are their "clients".

For the PC to initiate a sex session with normal NPCs, the target NPC must be drunk. This can be achieved by throwing alcohol on the target NPC (though this is usually considered an hostile action and, probably, a Karma penalty) or waiting for a character to become drunk on its own. Alternatively, giving (by interaction) an NPC an identified, uncursed bottle of alcohol will usually prompt it to immediately become drunk as well. Then, speaking with the NPC will show another option: "Interested in a little tail t'night?"

If your Charisma is high enough, the character and the NPC will exhibit a "heart" balloon (the same shown on an Ally upon killing), and time will go to Auto-Turn mode (as in Rest) with the Log showing flavor dialogue, consisting of the target NPC complimenting your prowess and, apparently, sex sounds. Duration of the act may depend on Charisma. When the event finishes, you will receive gold pieces as compensation, with an amount proportional to your Charisma, bounded by how much the NPC has or is willing to spend at once. You will also receive a bonus in your relationship with the target NPC. As such, prostitution is a good method to achieve money and raise favor with a vendor or adventurer without using love potions, going as far as getting "money back" from the purchase of an expensive deed.

Sex (and its relative abuse) may also lead to several negative effects, including Insanity and sickness. Each act of selling sex will give a -1 penalty to karma, unless you have used the Secret Treasure of the Wicked, in which case there is no penalty. This also trains Charisma for as long as the act continues, being a decent way to train the attribute if you do not have Performer, and have the means to remove any diseases you might contract.

Interrupted Prostitution

Your act of prostitution can be interrupted for various reasons, preventing any of the normal gains from prostitution, so prepare to avoid the following problems:

1. You will likely fail if your SP drops to tired level. (Tired level is at below 50 SP and prostitution takes about 50, so make sure that you have about 100 SP to be safe.)
2. Any damage will interrupt the act (including the occasional damage from being overweight or wearing cursed or doomed equipment). Generally, make sure that there are no obvious sources of damage around, including the likes of alien infestations.
3. NPCs that have no money to pay you, or are otherwise prevented from completing the transaction will exclaim "I-I don't really know that girl(boy). Please spare my life!". This prevents the collection of money, although you may or may not gain the other benefits.

Note that charisma is trained, and SP is consumed on a per-turn basis, and as such, interrupted prostitution still consumes SP. (This means you will be even more tired, and need even more rest if you try to initiate sex while tired, and fail because of it.)

Player Characters As the Buyer

Player characters can also have sex with prostitute NPCs, normally found in Derphy, in which case you are the "buyer" (unless the prostitute is drunk). Such acts do not require alcohol, but cost money, make your character insane, and carry the same risks of sickness. It does, however, exempt you from the karma penalty. This type of prostitution is beneficial mainly because it trains the player's will and constitution stats in addition to charisma, but repeated "relaxation" with a prostitute incurs the same chance to become diseased every time, and can cost a good amount of cash. 

Sex also depletes stamina for every round spent engaged in the act (which can last long enough to take up 50 SP, nearly enough to make you tired in one go if you have low Will or Constitution,) meaning you should rest (or sleep) to regain stamina between bouts.

NPC Prostitution

Prostitution can also occur between two NPCs. Both parties involved in the act won't be available for dialogue, showing the text "is in the middle of something", the same message that appears when an NPC is eating. 

In Derphy, sex between NPCs is a common sight. The prostitute NPCs that exist there can randomly engage in prostitution with other NPCs. This still apparently costs money from the NPCs, and if an NPC has no money when they "hire" a prostitute, it can (amusingly) lead to combat between the prostitute and the NPC. (Whom will likely be insane and incapable of effectively defending themselves from the prostitute until it wears off - although high-level characters like adventurers and trainers will likely brush off attacks long enough to recover their sanity and kill the prostitute.)

Because of prostitution, the prostitutes in Derphy tend to suck up all the money in the town rather easily, making them excellent targets for getting drunk to obtain their money, or using skills like Performer to get fat tips from them.

If the player owns a pet prostitute (via capturing a prostitute from Derphy), they can randomly engage in sex with other NPCs wherever you take them, and claim money from those characters. This seems to only happen fairly rarely, however. Like with other money that finds its way into the hands of a pet, you cannot directly take this money back, but it can be used to boost the potential of the pet when they are in a town with a trainer. You can also proposition sex with a pet prostitute whenever you wish (provided you have enough stamina) and pay them for it, transferring funds directly from you. Prostitutes can thus fund their own training more easily than other NPCs.

Note that if the NPC is wearing several cursed/doomed items or equipment with the "It sucks blood of the wielder"/"It causes random teleport" attributes, then the sex can be interrupted if the NPC's blood is sucked or (s)he warped away, resulting in no gold, karma penalty, or raise in favor, though it still consumes as much stamina as turns were spent in the act.