When you roll a 12 on the Special Features table, or if you just feel like it, you can select (or roll randomly) one of these.

Unless otherwise stated, delvers are of chaotic alignment.

Roll Special Feature
1 Surface War: An army of humans or humanoids, Black-round-mdBlack-round-mdBlack-round-mdWhite-round-md in number, starts at one edge of the map, and marches across the surface until they have an Encounter (which will usually be an Encounter with the Surface Kingdoms). If they find any tunnels, they briefly divert to explore them one finger deep. During their travels, they fight like a wandering monster and loot any unguarded treasure. After resolving their first Encounter, if any of the invading army survives, they build a border Fort. Draw the Fort, up to 3 beads in size as needed, near where the encounter happened. Each turn, if there are one or more Farms within one Finger from the Fort, roll a d6 for each. On a 5+, that farm is raided by the invaders, killing 1 Black-round-mdPeasant. The Fort ignores other Surface Kingdom buildings, and takes no other actions. Excursions, Adventuring Parties, and Monster Groups treat the Fort as if it were the lair of a group of Chaotic monsters. (Rolfe Bergström)
2 Minor Discovery: The group with the most Black-round-md discovers or creates a new feature. Roll on the Minor Features chart, and Draw the result on the map somewhere in their Zone of Control. If the Feature you rolled seems beneficial and appropriate, add either Black-round-md or White-round-md there. (If you are using the free version of How To Host A Dungeon, and thus don't have access to the Minor Features chart, re-roll or make up your own Special Feature instead.) (Rolfe Bergström)
3 Cave-In: The room or corridor closest to where the die lands becomes clogged with debris and is rendered impassable. Erase it or cross it out. If there were any creatures there, they lose one Black-round-md. If this breaks a Chaotic or Hungry groups' Zone of Control into two non-contiguous parts, the group splits into two groups, each with half the Black-round-md and Black-round-md of the original. (Rolfe Bergström)
4 Militarization: A random Delver or Breeder monster group starts producing weapons and training for war. Draw a Forge, Barracks or Training Grounds in their Zone of Control. They get a permanent +1 bonus on all Encounter rolls as long as that room is in their Zone of Control. (Rolfe Bergström)