Mistyra

Mysterious queen of the meat parasites, Mistyra reigns the Deserted Base as her own domain; she clearly lives in a symbiosis with her parasites, as they don't control them nor they bother her; conversely, she may order them and use as a tool or a weapon. She initially spawns when Lust Doll tries to leave her chambers with the key; afterwards, she may spawn in any third floor's room as repeatable random encounter.

The queen's domain
Lust Doll encounters Mistyra for the first time while trying to leave her chambers with the key from a locked exit door. She introduces herself, and hears Lust Doll's name in turn, but then states that she's not interesting in talking, nor she's planning to lest Lust Doll go, after which she immediately attacks. Lust Doll barely defeats her and escapes with their life, taking a, thinking that maybe local scientists would be interested in it.

Infestation
When whatever experiment the scientists were doing backfired horribly, and much of the research team got trapped on the upper floors, but from the exit by impenetrable meat wall, Lust Doll had no one else to ask for help, but the person with whom it all started: Mistyra.

Mistyra gets intrigued by her guest being so desperate that they're willing to ask for help even someone as dangerous as her, and agrees to provide some help. She shows them the meat core: the source of all infection, and its control centre. Destroying it would lead to a whole "hive" wither and die, but the core would defend itself, and the hive as strong as hers is almost invincible. However, the one in the laboratory is clearly immature and can still be destroyed.

Mistyra and Lust Doll goes to the laboratory together. Mistyra removes the meat wall, opening access to now-infested upper floors, and goes upstairs, ignoring protests of the lead scientist, Linda. Lust Doll follows her, accompanied by Linda, but when two reaches the top floor, they finds no traces of Mistyra; and when they tries to use the elevator, they gets attacked by the monstrosity created by fusion of the mad experiment and meat parasites; Lust Doll survives, but Linda is now gone – and so is Mistyra. Also, there's no trace of a core.

Linda's rescue
Lust Doll finds Mistyra already waiting for them on the upper floors of the Deserted Base. When pressed, she confirms that there indeed was no core, which took her by surprise just as much as it did Lust Doll: perhaps, the scientists somehow managed to make the meat spread without it. As for Linda, she was abducted by the meat parasites, and dragged to the even deeper levels of the base, to their nest; to save her, Lust Doll must go there – and Mistyra would follow them, not so much to help, as to see how they would handle it, and, maybe, make fun at their expense if they screws up.

The only way to make it into the nest is by squeezing through a meat tunnel, which you can't do while clothed, so Lust Doll has to undress first... and Mistyra just takes away their discarded clothes, apparently just for fun. The two travels together for a time being, and Mistyra even helps with opening a sealed passage by activating a meat orb, but quickly loses interest and leaves... without giving clothes back. However, much later, when Lust Doll and Linda finds another exit, Mistyra reveals that she was waiting for them there all this time, still keeping the clothes.

Mistyra reveals that firstly, she's not interested in Linda, seeing her as too weak to be of interest, and secondly, she doesn't care about what scientists did in the laboratory: even if they somehow managed to find a way to spread infestation without meat cores... just making more cores is easy enough to not look for alternate ways; not to mention, claiming more territories brings unwanted risk: you don't want to bite more than you can chew. Anyway, they both can go, for now.

Tactics
By default, Mistyra uses only physical attacks, sometimes skipping turns. Use Guard command when she smirks, or you would be ensnared. With you being bound, she uses the same normal attack, but with increased frequency, as well as would try to put on you the meat parasites if you don't have them already; those parasites acts the same way as they do normally: molests you at random, interfering with your battle efforts by causing you lust.

Electric Shock has stronger effect on her than on most enemies, due to her being infected, always draining 5% of her defence and making her skip turns, but be careful to not use it more than 3 times, or she would go into rage mod. When enraged, she starts using your parasites (if you're infected) against you, causing serious damage (chest parasite does heavy physical damage, crotch parasite causes mix of physical and lust damage), and occasionally makes devastating flail attacks, which can be fully blocked by using Guard command. She calms down on her own on 5th turn. Additionally, she can no longer be stunned.

It's possible to bond her by passing Strength check, which allows to use lust attacks against her, or, if you're strong enough, devastating physical attacks by squeezing her. While bound, she can't make attempts to ensnare you, but she still can attack you physically, and, if you're infected, can use meat parasites to molest you, causing lust. She may eventually break free, resuming fighting like normal.

Note that you can't rely on constantly running away from her; if you keep doing it, after several consecutive escapes she would catch up with you, resulting in battle starting anyway, with you being bound.

Post-battle scenes
Victory scenes
 * Defeating Mistyra while infected and choosing to pursue her would result in her trying to use parasites on your body to bond you to her will and forcing to give her cunnilingus, leading to Resilience 8+ check. Regardless of how things goes, she would remove parasites from the player afterwards.
 * Passing that check would allow Lust Doll to resist her, while interaction with parasites would lead Mistyra herself to orgasm.
 * Failing that check (or deliberately refusing to resist) would result in Lust Doll doing as ordered, but Mistyra would be disappointed with their weakness.

Defeat scenes
 * Losing to her would imprison Lust Doll in the chambers; see below for the list of scenes. If she's angry (see above), she would also humiliate the player before capturing them. Escape is possible, but not before she has her fun at least once.

Story events

 * During Infestation quest, losing to the Infected Abomination leads to a choice between two Bad Ends; one is abrupt, with Lust Doll becoming a part of meat mass, another leads to them becoming Mistyra's playtoy.
 * During Linda's rescue quest, the scene of squeezing through the meat hole to reach the nest plays differently, accounting for Mistyra's presence.
 * During Linda's rescue quest, Mistyra travels alongside Lust Doll, without helping in any way other than activating the very first Meat Orb, after which she promptly abandons them. While followed by her, losing to any enemy leads to one of the following scenes (note that all transformations are temporarily); they can be repeated during this short segment, but becomes permanently inaccessible afterwards:
 * Grow a megacock (cock TF and megacock must be enabled)
 * Grow megaboobs (breast TF and megaboobs must be enabled)
 * Molest body

Other events

 * Not escaping Mistyra's captivity after 4 cycles opens up a new option (always labelled as "???"); picking it triggers a Bad End.

Repeatable scenes and events

 * Being caught by the meat tentacles triggers special scene if Mistyra was encountered at least once, and female partners, tickling and feet shrines are enabled.
 * Being defeated or caught by the meat tentacles leads to her imprisoning Lust Doll in her chambers. While there, they may experience the following (note that options are marked as "???" until you try them for the first time):
 * Clit tease (req. pussy)
 * Breastjob (req. cock)
 * Tickle and edge
 * Transform to female (req. no pussy)
 * Grow cock (req. no cock)
 * Milking feed (req. )