The vault guards are under the employment of Croesus, and are the source of the footsteps you hear on some levels, indicating there is a vault on that floor. Guards are mercenaries similar to Yendorian Army soldiers, and can even be bribed.
As mercenaries, guards follow similar generation mechanisms to soldiers and watchmen for armor and weapons; see that article for more details. Vault guards will also carry cursed tin whistles, providing a source for the shrill whistling sound a player will hear while escaping them.
Guards are not randomly generated, and only appear if the player waits around while within a vault, creating an opening in the vault wall or the outside solid rock for them to stand in. Vault guard generation increments the born counter, but does not respect extinction; while there is no limit on how many guards a vault may generate, only one will appear at a time.
Interacting with guards
If you linger within a vault for 30 turns, a guard will enter and ask for your name; blowing a cursed tin whistle while inside a vault will immediately summon a guard. If you reply with Croesus, Kroisos or Creosote, they will believe you and disappear - this incurs a small alignment penalty for lying if you are lawful, unless it is also your actual name, and attempting this after killing the actual Croesus will make them hostile. If you attack the guard or attempt to escape by some other means, they will also turn hostile. Killing a guard will count as murder, even if the guard attacks you first.
If you answer their query with a different name, they will ask you to drop all of your gold and follow them out of the vault - this includes any gold you have in open inventory and in containers within your inventory, so stashing cannot fool them. The guard will escort you out as long as you comply, and this can be used to escape a vault if you lack any other means to do so - if you pick up the gold again, they will demand you drop it and turn hostile if you do not do so quickly. However, you can freely teleport it away or throw it outside of the vault without angering them.
Once they see you have no gold, they will open a passage from the vault to a nearby corridor in the dungeon and wait for you to follow them out - both the guard and passage will vanish once you safely reach the corridor, and you can return later with a means of digging or teleporting to retrieve the gold. Items left inside the passage will be embedded in the solid rock; any other monsters caught in the disappearing passage will be moved to a random location on the current level. If a guard dies in the passageway they create, it will close immediately and their corpse and possessions will be left embedded in stone, along with you if you were in the passage.
If a guard appears and you are in a polymorphed state from eating a mimic corpse, or are polymorphed into a form that can hide, they will not notice you and leave while puzzled. Doing so while hallucinating produces a different message, but otherwise has the same effect. If you are unable to speak, e.g. due to paralysis, unconsciousness, or else being in a form that cannot speak, they will also leave. The guard will not notice you if you are invisible when they enter, and will leave when you move away from them.
Guards have the same speed as an unhasted hero and armor that usually places their AC at 0 or less - however, their sole weapon attack can hit for up to 40 base damage, outperforming even Yendorian captains. This is mitigated by the fact that guards are very easy to avoid fighting, and are usually not worth the effort most times. Once players can reliably make their way in and out of vaults, e.g. via digging or controlled teleports, the guard is little more than a minor obstacle at best.
Some players name their character Croesus (or one of the acceptable variants) to avoid the alignment penalty for lying, though said penalty is small and easily remedied. While guards can be bribed, this is often a waste of gold even when successful, but can be of use in some corner cases, e.g. to pacify a hostile guard and avoid murder penalties.
Chaotic characters are not penalized for murder and can theoretically farm guards for items, but will still suffer a massive alignment penalty. Furthermore, the items a guard will drop are easily accessible by pilfering any barracks that you encounter - as such, guard farming is usually only of interest to extinctionist players, as well as chaotic players who have somehow exhausted any other reliable source of items and can somehow soak up the alignment penalties.
Should a player dealing with a guard somehow get stuck in a disappearing passageway, they can pray to escape; being stuck in a wall is considered a major trouble, and a successful prayer will teleport them out. If teleportation somehow fails, the player will be granted temporary phasing for a handful of turns, which should be enough for them to make it back onto a safe square.
- Suddenly, one of the vault's guards appear!
- A guard entered the vault while you were present.
- "Hello stranger, who are you?"
- The guard can see you; this will give you a prompt to enter your name.
- "Most likely all of your gold was stolen from this vault."
- The guard is aware of the gold in your inventory and wants you to drop it all.
- "Please follow me."
- The guard is leading you out of the vault, opening a passageway through the solid rock if needed.
- "Drop all your gold, scoundrel!"
- You attempted to pick up the dropped gold again while in their view.
- "So be it, rogue!"
- The guard becomes hostile because you did not drop your gold in time or else refused to.
- You hear the shrill sound of a guard's whistle.
- You fled from a guard without dropping your carried gold while they were watching you, and they blew the whistle in their inventory.
- You hear angry shouting.
- As above, but there was no whistle in their inventory.
- "Oh, yes, of course. Sorry to have disturbed you."
- You told the guard you are Croesus, causing them to leave.
- "Back from the dead, are you? I'll remedy that!"
- As above, but you have already killed Croesus; the guard will become hostile.
- "I'll be back when you're ready to speak to me!"
- You were in an impaired state that prevents you speaking when the guard entered, and they left.
- The guard turns around and leaves, puzzled.
- You were not visible when the guard entered, causing them to leave immediately. This is the result of eating a mimic corpse or polymorphing into a hiding monster and hiding under the gold.
- "Hey! Who left that orange in here?"
- As above, you were polymorphed from eating a mimic corpse while hallucinating when the guard entered.
- See the encyclopedia entry for human.