How does an NPC set up unobvious long-distance alerts similar to Alarm?

Question

I have a 7th-level wizard NPC who would like to be secretly alerted if someone enters an underground location that is important to him. He will always be within 20 miles of the location. He would like to set up this "alert" once and then have it last until triggered; he would not want to have to return to the location to re-cast any spells very frequently. (Returning once a year or so would be OK.) Ideally, the intruder would not be aware that anyone is monitoring this location. The wizard has a normal amount of resources, so no solutions which cost zillions in gold. Is there a way to do this, without homebrewed spells/items?

Ideas I have considered (and rejected, with reasons):

1. alarm. Problems: only lasts 8 hours. Also the mental "ping" has a range of only 1 mile, but the duration is the dealbreaker here, otherwise it'd be perfect.
2. Cast glyph of warding with a trigger of "someone enters the location". Cast skywrite [EEPC] into the glyph, to write "Intruder!" or similar into the sky when the glyph is triggered. Problems: the location is underground and so the sky can't be seen from it, and skywrite requires the caster to be able to see the sky. Also has the problem that other people can see the "alert" in the sky, although what's written could be made obscure to make this less likely. It's also not all that efficient; our wizard has to look at the sky pretty regularly (how often do you look at the sky?)
3. A magic mouth "telephone wire" (discussed at GitP). Basically, you cast a chain of magic mouths every 30ft between the location and the wizard, where the first one is triggered by an intruder and whispers "Intruder!", and all the others are triggered by the previous magic mouth in the chain whispering "Intruder!" and then say "Intruder!" themselves. Problems: costs a lot of money (10gp per mouth), and it only works to a fixed location; the wizard has to remain at the end of the (static) chain forever. It's also possible that other people may hear the chain speaking, and the intruder themselves may also hear it and thus be alerted that their intrusion has been detected.
4. Create a keycharm [E: RFTLW]. Cast glyph of warding, with condition "an intruder enters the area" and tie it to the keycharm. Cast alarm into the glyph, tying it to the keycharm. If an intruder enters and avoids the glyph, that will trigger the keycharm; if the intruder does not avoid the glyph, then the glyph will cast alarm, the intruder will trip the alarm, and that will trigger the keycharm. Problems: you need to be or have the help of a dwarf with the Mark of Warding from Eberron.
5. Station your familiar at the location, and have the warlock eldritch invocation voice of the chain master so the familiar can tell you about an intruder from any distance. Problems: you need to be a third-level Pact of the Chain warlock with voice of the chain master, and then give up your familiar to have it guard the location, which is a pretty heavy cost to pay.
6. Find a friendly awakened shrub, or create one with a pot of awakening [XGTE] (the awaken spell is too high level). Obtain a pair of sending stones [DMG]. Since the shrub is friendly, and has human-level intelligence (10), it should be able to obey commands, and since it's a living thing that can take actions, it should be able to use an action to activate the sending stone. The shrub is defined as knowing "one language known by its creator", but it's not clear whether it merely understands that language or can actually speak. If it can speak, it can warn the wizard by sending through the sending stone. If it can't speak, then request it to use its action when it spots an intruder to activate the sending stone and then wave in the air; put a magic mouth on its pot with trigger "when the plant waves, say 'Intruder!'", and then hopefully the magic mouth can be heard through the sending. (Problems: This second approach is rather more dubious! It's not at all clear that one creature can cast sending and then have someone else do the speaking. And it's quite likely that the shrub can't speak and thus can't send anything through the sending spell, even if it can use an action to trigger the sending stone. However, the awaken spell does gift the shrub the ability to speak ("The target also gains the ability to speak one language you know"), so presumably the pot of awakening does as well.)
7. Have a magic mouth trigger on an intruder and use a sending stone. Problems: I'd rule against this as a DM; the stones require an action to trigger, and magic mouths can't take actions.
8. Cast contingency, with trigger "an intruder enters the location", and have it do something to you, wherever you are. Prestidigitation to make sparks appear in front of your eyes briefly, say, to warn you of an intruder. This is the perfect solution but it's a 6th-level spell and thus unavailable to our 7th-level wizard.
9. Rube Goldberg approaches which involve a glyph of warding (somehow) destroying one of a pair of sending stones, thus rendering the other one (in the wizard's possession) instantly nonmagical. Problems: I don't think there's a way to be alerted that a stone in one's pocket has suddenly become nonmagical, plus ideally one would not destroy a magical item just to make this alarm work.

There are obviously homebrew or unlikely approaches (find or buy a scroll of contingency, or craft a dedicated magical item to do this), but I'm trying to find an approach without crafting a homebrew item to basically cast permanent alarm (since that's basically DM fiat in allowing it, and I prefer to have my NPCs not just pull powers out of their hats but instead follow the same rules as the PCs, when possible.) I will of course do this if necessary, but I'm hoping there's a more creative way.

A similar question was addressed at How can I set up a long-range alarm for my fortress? but this example is more constrained; the wizard here will remain closer to the location, and is lower-level (and therefore is happy to accept more constraints on the solution). But "tell me if someone enters this area, even if I'm some distance away" does not seem too outlandish a thing for a 7th-level wizard to be able to creatively do.

Answer 1

If the loss of a familiar is acceptable: Find Familiar (Raven) and Sending Stones.

While a familiar is unable to speak your language the Raven does have the Mimicry ability.

Mimicry. The raven can mimic simple sounds it has heard, such as a person whispering, a baby crying, or an animal chittering. A creature that hears the sounds can tell they are imitations with a successful DC 10 Wisdom (Insight) check.

Should your DM be willing, you can teach it a word or two to speak into a sending stone (DMG 199). (The only action prohibited to it is the attack action).
If your DM is not open to you teaching words to your Raven, then I'd recommend coded "caws", two means a visitor presenting some seal of visitation. Four means an intruder. 1 or 3 would then mean an intruder willing to silence your bird. If relevant the Raven can fly around, allowing it to spot intruders from a distance or at more than one entrance.

Answer 2

It's an NPC, so you can create an effect specifically for them, or change the game in ways that don't meaningfully change PC options.

I would modify Alarm with a clause like Guards and Wards

You can create a permanent alarm by casting this spell in the same location every day for one year.

Answer 3

This one was tricky, but I think I have a solution

First, the things that didn't work...

1. Sequester; cast it on any minion and trigger them to wake up when someone breaks in. Problem is, it's a 7th level spell, so too high.
2. Create Homunculus; this was my favorite. It's a construct so it can just sit there forever waiting and guarding, AND it has built-in telepathy! "While the homunculus is on the same plane of existence as its master, it can magically convey what it senses to its master, and the two can communicate telepathically." Unfortunately, it's also a 6th level spell so too high.
3. Divination; getting closer. One a day, you can ask a deity about an event that will occur within 7 days. So every morning wake up and ask your god, "Will someone break into my personal space in the next week?" It's like having your own precogs for an alarm. But it might wear thin after a while plus it's a Cleric-only spell.

So what was the final solution?

Glyph of Warding and Sending

Both are third level and can be cast the same day by a 7th level wizard.

So cast the Glyph so that when it's triggered, it casts the Sending spell. You have a pre-programmed message telling you that you have an intruder.

Sending "can send the message across any distance and even to other planes of existence," so you're not tied down anywhere.

But there is a rub; per the Glyphs spell, "If the spell has a target, it targets the creature that triggered the glyph" which means that the sending spell will send a message to the intruder, NOT the NPC (unless the DM says it can work that way). But there is a way around this. The person tripping the ward "can answer in a like manner".

So have the message be a question, or a riddle.

"Halt! Who goes there?"
"Only someone worthy may pass. Answer my riddle; What is brown and sticky?"

The NPC will get a message, anywhere, anytime, with the answer so they can act accordingly.

Answer 4

• The wizard has hired a minion with a scroll or wand of sending. The minion has enough use magic device to cast the spell.

Cheaper option:

• The wizard has hired a minion. The minion also has a horse. When he sees the pcs enter the building he uses the horse to ride to the wizard and tell him.

Drawback: Would take I think 4 hours or so to reach the wizard.

Even cheaper option:

• The wizards have hired two minions. One to keep guard on location, the other to keep guard at the wizards location. Minion A lights a signal fire. Minion B keeps an eye out for signal fires and notifies the wizard. (according to google a signal fire can be seen for 30 miles) This approach also works with skywrite. Drawback is it's less secret than the two above.

Answer 5

Combine a Glyph of Warding with any spell that grants the caster information

For example, Glyph of Warding charged with Zone of Truth: the Zone of Truth spell specifies that "You know whether each creature succeeds or fails on its saving throw." Thus, when the glyph is triggered, you will know that it was triggered because you will know whether the creature triggering the glyph failed its saving throw. For Zone of Truth in particular, the creature that triggers the glyph will also know ("An affected creature is aware of the spell"), but a spell like Find Traps, which tells you about traps in the area of the glyph, would suffice. Clairvoyance would also work (assuming you are within a mile) or Arcane Eye, as the location of the glyph would be the target, and you would be able to see the intruder.

The problems with the above spells:

• Zone of Truth: not a wizard spell, and the intruder is aware
• Find Traps: not a wizard spell
• Clairvoyance: must remain within a mile
• Arcane Eye: must be level 8 to cast with glyph of warding (2 L4 spell slots required)

Other spells that might work:

• Find Familiar: "If you cast this spell while you already have a familiar, you instead cause it to adopt a new form" or your familiar would be summoned near the glyph if you didn't have one already
• Identify: you discover what spells are affecting the intruder
• Wither and Bloom: "one creature of your choice in that area can spend and roll one of its unspent Hit Dice" so it's debatable whether you would know about this
• Life Transference: causes you damage when the glyph is triggered and heals the intruder, so less than ideal, but effective
• Vampiric Touch: debatable whether a glyph would grant the spell benefits to the intruder or target the intruder with an attack, but in the latter case, you would be healed
• Animate Dead: this grants you a mental link to the raised undead through which you can make commands, but it's unclear if you would be aware that this happened were the spell to be triggered by a glyph

Of these, I would judge Identify as the most likely to work as intended for an L7 wizard; it should grant the caster knowledge of what spells are affecting the intruder when the intruder triggers the glyph, and the intruder should be none the wiser. Of course, one can always debate whether Identify works with Glyph of Warding, as this isn't clearly answered in the rules.