Mob Siege

Mob Sieges are a feature added in Minecraft 1.18. When a mob siege occurs, hostile mobs will converge on the player's last slept-in bed and besiege whatever structure(s) block them from reaching it, similarly to a zombie siege or raid for a village.

Which mobs will spawn for a siege depends on which biome the player's bed is located in. Some biomes, such as Between-space, are specifically ineligible, even if beds can be placed there. Most hostile mobs that can be found roaming the overworld, whether common (IE: Spider) or biome-locked (IE: Husk) participate in at least one variant of mob siege, usually only for their home biome in the case of the latter. The vast majority of rarer mobs (IE: Enderman), non-spawning mobs (IE: silverfish), and structural mobs (IE: mummies) do not participate.

Occurrence[]

Mob sieges require the following global conditions to be met in order to occur:

The world must be on Hard difficulty;
There must be at least one valid target (see below);
It must be night time if in the Overworld (the raid can continue during the day, but can only start at night);

In order for a player to be considered a valid target, they must:

Have a valid bed - This is a bed which currently exists, is placed in an acceptable biome, and has been slept in by them within the past 5 in-game days, and is in a loaded chunk. If more than one player has slept in the same bed, it will only count for the last player to sleep there;
Have regional difficulty of over 5.5;
Be in-game at the time of the siege;

If all of these conditions are met, on survival/adventure mode, the game has a 3% chance of sending a mob siege to each player's house. The percentage chance will go up 1% for every .2 regional difficulty point the player has (a maximum of 9% for players with the highest regional difficulty). There is a cap of five sieges per night, and one siege per player.

Hardcore mode has different rules, with a 20% base chance for valid targets, plus 1% for every tenth of a point of regional difficulty (a maximum of 32% for players with the highest regional difficulty). The cap of one siege per player remains in place, but there is no limit on how many players can be besieged.

Naturally, with the exceptions of mods or glitches, the player can only experience mob sieges in dimensions where a bed can be placed. In the Nether or the End, beds will explode upon being used, making them ineligible to be targeted.

Variants[]

The mob siege has a large array of variants that are used depending on the biome and dimension. Variants determine which mobs spawn.

Temperate Dry[]

The "temperate dry" variant is used for all overworld oak/birch/dark oak forest biomes and all grasslands biomes.

Cold[]

The cold variant is used in tundras, taigas, and other cold, terrestrial, overworld biomes.

Desert[]

As the name implies, there is a mob siege variant for deserts and mesa biomes.

Jungle[]

The Jungle variant is used for jungles and the jungle edge biome. This variant is meant to account for treehouses, and its mobs have a focus on high vertical mobility or ranged attacks.

Swamp[]

This variant is used for all swampy overworld biomes. Common mobs are missing, probably due to their tendancy to get stuck and slowed down in the water channels of swamps.

Hills[]

The Hills variant is used for mountainous biomes, including all variants of extreme hills. Rockmen are not part of the mob group here, but will often join in if a player has not properly sealed up caves in the area, as their natural behavior is to emerge from caves, then seek out and attack players from quite a distance.

Ocean Normal[]

The "Ocean Normal" variant is used for all variaties of oceans and beaches.

Burn[]

Simply referred to as "Burn" in the game files, this mob siege pack is used for the Burnt Down Oak Forest and Volcano biome. The only mob aside from the sapper zombie is the Charred Zombie.

Corruption[]

This mob siege is exclusive to the Corruption biome, for the rare player to make their home in such a hostile place.

Deep Woods[]

For the Deep Woods biome.

Villager Revenge[]

This variant only occurs if the player has a very bad reputation with a village, and there is still at least one villager left alive who is part of that village. It can occur in any biome and overwrites the normal biome variation for that night.

Unused Variants[]

Some variants remain even though no beds could ever be used in their biomes. It's possible a change to bed mechanics was planned, or the mob sieges were originally intended to target something other than the player's bed.

Nether - The Nether variant includes blazes, magma cubes, wither skeletons, and Nether Knights. This works as normal if the player gets the Hell biome into the overworld via superflat preset.
End - The end variant only includes two mobs, the Ender Warrior and Ender Slime. It's apparent that this variant is incomplete. This works as normal if the player gets the "The End" biome into the overworld via superflat preset, but does not work on other end variant biomes, such as the End Barrens or Ender Depths.
AAA_TEST_1 - Presumably used for testing. This variant includes, zombies, phantoms, drowned, spiders, and, perhaps strangely, Grubbers.
Slime - This just includes slimes of varying sizes. What the intended purpose of this variation is, if any, is unclear. It may have been intended for the swamp, or a scrapped new biome.
Unfortunate - This unused variant consists of charged creepers, Ballistic Creepers, Pink Creepers, Creepowls, and Creeper Head Swarms.

Additions[]

On top of the normal variant, some special conditions can cause certain mobs to spawn.

If the targeted bed is above Y=150, skirons can spawn and attack the player from above.
If the targeted bed is below Y=35 and NOT exposed to sunlight, drillers will spawn and burrow down.
If there is a nether portal within 100 blocks of the targeted bed, a small number of blazes and/or magma cubes can spawn out of it.

Strategies[]

Farming[]

It is possible to use mob sieges to farm a wide variety of mobs. If the player leaves an easily demolished wooden door entrance, the mobs will almost always flood it immediately without bothering to look for harder routes in. If you can pick off the Sapper Zombies via bow and arrow, it is easy to funnel the remaining mobs into grinders.