Kill hole
A kill hole (murder hole) is a type of wall formation that only allows one monster into your line of sight at a time. Since most monsters can't do anything when outside your sight, killholes are an improvement over a regular corridor. In a hallway, an orc priest might smite you from behind other orcs. But in a killhole, it is unable to see you, and thus unable to attack.
Kill holes may occasionally form naturally, but most mentions of them refer to the creation of artificial kill holes with a formicid's digging ability or a wand of digging.
How It Works
In short, a killhole is the intersection of 2 diagonal lines ('V', '^', '<', or '>' shaped). Cardinal directions do not work - see below for a demonstration.
X's show line of sight radius.
Left: Not a killhole; priest can attack from 2 tiles away
Right: Killhole, priest forced in melee.
Natural Examples
######### ####.#### ###.#.### ##.###@## <--stand on ..#####g# the @ ##.###g## ###.#g### ####g#### ####.####
This is an example of a naturally formed kill hole you might see in the Dungeon or Elf. Do note that it allows monsters to come from behind. While these gnolls are unlikely to find the other entrance, other monsters may hear the noise and approach from the west.
.###### ..YY#.. .YY+@.# <--stand on ..##### the @ #######
Another example, this time of something you might see in a semi-common layout of the Lair. In this example, a door (the + between yaks and player) is blocking enemies through. If an enemy can open doors, this kill hole won't function correctly. Also, like the previous example, creatures may still show up from the east to attack you from behind.
Creating a kill hole
To make your own kill hole, you'll need a section of rock wall, and a way to dig through it (formicid ability, or wand of digging). Ideally, the wall you're digging into won't have any open spaces behind it that you might break into by accident, which can open a whole new can of worms. The edges of the floor are usually safe, but if you're too close to the edge of the map, you might run into the indestructible border walls.
While Shatter can destroy walls (even walls made of more durable substances like stone, metal, or crystal), it lacks precision. You might get lucky and make a kill hole, or you might just break the entire wall down, and either way, you'll be making an incredible amount of noise.
A perfect kill hole (one which only ever allows one enemy into your LOS) looks something like this:
#### .#@# #.## ####
Formicids can dig walls one at a time, so this configuration would be trivial. In contrast, a wand of digging will always dig multiple tiles in a direction. You can still make a perfectly functional killhole, though there'll be a space behind you.
To begin, find a suitable patch of rock wall:
############# .############ ..########### ...########## ..@########## ...*######### <-- Dig pointing at ...########## the asterisk ...########## ..########### .############ #############
Now, take a couple steps into your new tunnel, then dig diagonally in the other direction:
############# .############ ..########### ...########## ...########## ....#*####### <-- Dig pointing at ...#@######## the asterisk ...##.####### ..####.###### .######.##### ########.####
Stand just inside the entrance to the second tunnel, and congratulations, you've got a kill hole!
##########.## .########.### ..######.#### ...####.##### ...###.###### ....#@####### ...#.######## ...##.####### ..####.###### .######.##### ########.####
Having that bit of extra space has a few minor downsides. Enemies with blink can end up behind you. Hostile summoners can summon monsters in that space, too. Otherwise, it works just like the "perfect" killhole as mentioned above.
Tips & Tricks
- Kill holes are best used when you know there are monsters nearby with dangerous abilities that rely on sight; dangerous summons, powerful ranged attacks/spells, Smiting, Damnation, and Torment are all good examples. If you alert a number of monsters with these abilities while exploring, you may wish to retreat to a suitable kill hole (or quickly build one, if there aren't any nearby).
- If you know there is a vault with monsters of this sort on the current level, you may wish to preemptively build a kill hole and retreat there after attracting the attention of creatures in the vault. Particularly good places to do this include Orc:2, the ends of the various Lair rune branches, and Zot:5. (Elf:3 used to be a good place to do this before deep elf elementalists were introduced; see below.)
- Kill holes prevent monsters from seeing you, but they also prevent you from seeing monsters. Try to pay attention to any messages that mention noises coming from behind whatever you're fighting, as that might announce the presence of a monster you hadn't known about when you retreated. Formicids have yet another advantage in their ability to sense creatures with their antennae.
- Always make sure you have an escape plan. Even with the advantage of facing your enemies one at a time, you may get unlucky and end up overwhelmed and backed into a corner. Alternatively, something far more dangerous than the enemies you were fighting might be attracted by the noise and suddenly appear with little to no warning. A scroll of teleportation is perfect for escaping -- just be sure to leave enough time for it to kick in.
- Some monsters (like deep troll earth mages or cacodemons) have a Dig spell of their own, as do any creatures that spawn with or pick up their own wand of digging. Any of these will happily break you out of your carefully prepared bunker and open paths for their allies to swarm you. Deep elf elementalists are even worse, as their Awaken Earth spell turns nearby walls into hostile earth elementals; don't rely solely on a kill hole to survive Elf:3.
History
- Prior to 0.22, Lee's Rapid Deconstruction could destroy walls, possibly enabling the creation of kill holes in stone, crystal, or metal structures. With a bit of luck, it was possible to make a kill hole even in Tomb:3.
- Prior to 0.20, wands of disintegration could be used to remove one tile of rock wall at a time, giving all characters access to formicid-level precision in making kill holes.
- Prior to 0.14, players had access to the Dig spell.
