Kill hole
A kill hole (sometimes known as a murder hole) is a type of wall formation that only allows one monster into your line of sight at a time. This is an improvement over simply luring enemies into a corridor because most monsters will not do anything when they cannot see you; gnolls cannot reach you with polearms, orc priests cannot smite you, kobold demonologists won't summon any demons, necromancers won't shoot bolt spells past their zombies, etc.
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.
Examples
######### ####.#### ###.#.### ##.###@## <--stand on ..#####g# the @ ##.###g## ###.#g### ####g#### ####.####
This is an example of a naturally formed kill hole you might see in the Dungeon. Notably, it does still provide monsters with a way to get behind you -- while the gnolls chasing you from the south are unlikely to find it unless there are a great many of them, any other creatures attracted by the noise or that are simply wandering may come around 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. Do note that this kill hole makes use of a closed door (the + between the yaks and the player) -- when dealing with monsters that can open doors, this kill hole would not 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 to have a section of rock wall to work with and either be a formicid or have a 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, possibly attracting the attention of anything dangerous inside. The walls at the edges of the floor are usually perfect for this (though if you're too close to the edge of the floor, you might run into the indestructible border walls), but you can make do with digging into a large pillar or the like.
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, with their ability to dig into walls one space at a time, can easily make winding tunnels where each individual tile is its own kill hole. Of course, other characters will only have a wand of digging at their disposal and won't be able to make anything nearly so precise; they can, however, make a perfectly functional kill hole with only a little extra empty space behind them. 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!
##########.## .########.### ..######.#### ...####.##### ...###.###### ....#@####### ...#.######## ...##.####### ..####.###### .######.##### ########.####
The only downside to making a kill hole with a wand of digging is if any creatures you're fighting have the ability to blink. They may end up blinking into the empty space behind you, leaving the path clear for a second enemy to get at you. Similarly, hostile summoners can summon monsters behind you.
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.
