Pacifying levels refers to stopping further monster generation by allowing a section of the level to fill up with bred monsters, until the level stops generating new ones. The magic number appears to be 40 — once there are forty monsters on a level, no new ones will be spontaneously generated — although breeders and summoners will still produce monsters as long as they have access to open space.
In ADOM 1.2.0 prerelease 23, 40 monsters was not observed to be enough to pacify a level. Monster generation did not stop until 80 monsters were present. In release 60, 105 monsters was not enough to pacify the Big Room but 195 was; might the new number be 120 or 160?
For the same reason that many players pacify the Big Room — to allow herbs to be harvested peacefully. But it's generally much easier to pacify other levels, so as long as another herb-growing level can be found, it's not necessary to spend the extra time to pacify the Big Room.
On regular levels, this is as simple as finding a big enough room, or a pair of smaller rooms connected by a corridor, or digging out a smaller room until there are at least forty floor tiles, and then leading a breeder there. Run or teleport away and close the door. Since worms, insects, jellies, and the like can't open doors, it may not be strictly necessary to lock them up, although a closed door may potentially be opened up by another passing monster before the level is pacified. Relatedly, on deeper levels or with higher-level PCs, a monster capable of smashing locked doors (such as a hill giant) may spawn inside the room. Therefore it may be a good idea to stay by the door with a wand of door creation until monster generation stops completely. It may also be necessary to enter the enclosure to kill non-breeder monsters in the event that they block a corridor.
It appears that, at least on regular (non-cavernous) levels, monster generation slows significantly when the level has only a fraction of the forty monsters needed to stop it completely, and since the growth rate of breeders is exponential, it takes only a short time once a breeder is found before the rest of the level is completely peaceful, and more herbs can be planted.
- Brown worms
- Chaos rats
- Chaos spawn
- Claw bugs (and greater claw bugs and killer bugs, although you're not likely to see the latter two on levels where it makes sense to farm herbs)
- Gray oozes
- Green blobs, green worms, and green slimes
- Ochre jellies
- Purple worms
- Red worms
- White worms
Note that other insects (like fire beetles) are not breeders, nor are other jellies (like gelatinous cubes, gibbering mouthers, yellow oozes, death oozes, and stone oozes). The monster symbol doesn't necessarily tell you whether it will breed, although all of the worms (represented by "w") do.