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My wizard is building a large, magically defended fortress. I have many powerful wards and defenses protecting it, but there's one large weakness - if I'm not in my fortress, I have no way to tell if someone is trying to break in. I'd like a way to be alerted that there are intruders in my fortress while I'm out saving the world. This can consist of a spell, a series of spells, or any other complicated shenanigans involving spells and magic items as needed.

Here are my specific requirements:

  • I need the alarm to be activated by the presence of creatures in a specific area. Ideally it would work like Glyph of Warding where you can set specific conditions and exceptions, but I'd be willing to accept an alarm that simply triggers if anything enters the area.
  • The alarm needs to be able to alert me over long distances. Ideally, it would at least reach anywhere on the same plane. If such a solution doesn't exist, an alarm that I can detect over distances measured in hundreds of miles would work.
  • The alarm needs to be long-lasting. Ideally it would be permanent, but at bare minimum it needs to last for a month before I need to return to my fortress.
  • The alarm signal needs to reach me relatively quickly. I need to have time to get the message, teleport back to my fortress, and confront the intruders. A signal or message that takes days to reach me will not meet this requirement.

Here are my constraints and resources:

  • Anything in DnD Beyond that is not adventure-specific is legal - no homebrew or UA
  • Assume the spellcasting ability of a level 20 Wizard with any spells on Wizard list available
  • Assume I can obtain the limited services of a level 20 spellcaster from another class (i.e. I could call a favor to have a Cleric set something up once, but I can't have them come by and refresh the spell every week)
  • Monetary cost and material components are inconsequential, although lower cost is obviously preferred
  • Limit to six months of prep and setup work
  • Use of magic items from the DMG as part of the alarm system is permitted, but limited to one item that requires attunement
  • Assume I have the resources and companionship of several other fourth-tier characters (level 17-20) to help if I need to undertake a particularly dangerous quest for a component of the alarm system

If no answer meets all of these requirements and constraints, please come as close as you can. I've spent some time trying to puzzle out a way to make this work and I'm stuck - it's easy with a fairly reasonable homebrew, but within the constraints of spells and items we already have, I can't see a way to make it work.

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  • \$\begingroup\$ I know that Wizards are typically paranoid by nature, because they know things... but how many times in the course of your month long holiday to you expect to have a breach attempted? \$\endgroup\$ – Slagmoth Mar 27 '18 at 13:11
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Leave a Simulacrum of yourself guarding the fortress

  1. Cast a Simulacrum of yourself
  2. Provide it with a copy of your spellbook
  3. Have the Simulacrum keep ritual casting Alarm in the relevant areas.
  4. The Simulacrum can cast a Sending to you when the Alarm is triggered.

This has a setup time of 12 hours plus however long it takes to copy the spells you want to provide your simulacrum and needs to be renewed when the Simulacrum runs out of slots to cast Sending.

Alternatively you can provide your Simulacrum with a more easily renewable Sending resource:

  1. You can give your Simulacrum a ring of spell storing which you can renew yourself.
  2. Provide the simulacrum half a Sending Stone set (as suggested by Slagmoth)

Other considerations

Your Simulacrum can keep casting other rituals spells you provide it.

This expends your single simultaneous Simulacrum limit unless you use a method such as described here.

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    \$\begingroup\$ Alternatively, provide the simulacrum half a Sending Stone set. \$\endgroup\$ – Slagmoth Mar 27 '18 at 13:06
  • \$\begingroup\$ @Slagmoth I've gone ahead and included that though I don't have my DMG here so if you think that aspect needs further clarification feel free to note it (or edit yourself if you feel like it) \$\endgroup\$ – Sdjz Mar 27 '18 at 13:28
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    \$\begingroup\$ Remember that a simulacrum is friendly to you, but it is also a duplicate of you. So it would find repeatedly casting tedious spells and hanging around alone in a castle exactly as boring as you would; and it would want to go out adventuring exactly as much as you would. \$\endgroup\$ – Marq Mar 27 '18 at 16:29
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    \$\begingroup\$ @Marq that is a valid point to consider but you can force it to stay behind and guard no matter how bored it gets since "It obeys your spoken commands, moving and acting in accordance with your wishes". Any side effects of prolonged boredom are best left to a conversation with the DM though, I think. \$\endgroup\$ – Sdjz Mar 27 '18 at 17:38
  • \$\begingroup\$ I think this is the answer I needed, and actually opens up a wide range of possibilities. I could also summon a creature and bind it with Planar Binding, or construct a golem. I think the key I was missing was "Intelligent, magical creature with sending stones". Doesn't need sleep or food, and perfectly capable of standing watch in one spot for months on end. \$\endgroup\$ – Dacromir Mar 27 '18 at 19:59
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Hire guards who can cast Sending.

Sending works for any distance, even across planes. It has a 5% chance of failing across planes, but since you get to answer, the spell failing can be noticed and compensated by repeated castings.
Hire multiple guards and pay them well to reduce risk of defecting or them getting taken out before being able to send a message. If you cannot find suitable magic users willing to waste their life in your fortress maybe compensate them by giving them limited access to your library. Alternatively acquire some Item of Sending so you can just hire commoners.
As far as I can tell it meets all your requirements and is very cheap on resources compared to what you are willing to pay.

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  • \$\begingroup\$ Guards are a step in the right direction, but leave concerns associated with mortality (boredom, falling asleep, needing food, deciding to leave, etc). However, including an intelligent creature as part of my system is definitely the point I was missing. \$\endgroup\$ – Dacromir Mar 27 '18 at 19:59
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Depending on the climate around your stronghold, another very inexpensive possibility would entail:

  1. A nice forbidding swamp surrounding your keep

  2. Lots and lots of frogs living in said swamp

  3. A faithful Bullywug manservant with good hearing

  4. A pair of Sending Stones, as per @Slagmoth's suggestion in a comment

The Bullywug can naturally speak with Frogs and Toads, allowing for an all-natural alarm system surrounding your fortress (can't be dispelled, and unlikely to even be noticed by an intruder). Your bullywug then only needs to croak at you through the sending stones when anything unusual wanders into the swamp and gets the frogs gossiping.

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  • \$\begingroup\$ +1, this has the added benefit of being able to tell intruders to "get out of mah swamp!" \$\endgroup\$ – Josh Jan 10 at 14:55
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Use guards and wards in conjunction with a sending stone.

Place a suggestion that who ever enters (who is not you) pick up the stone of sending, activate, and declare "Visitors have arrived." The other part of the suggestion is that they then return the sensing stone to its proper space

As guards and wards can be made permanent, and sending some when's so long as the wizard us in the same plane, this should work.

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You could cast glyph of warding, storing a sending spell with you as the recipient.

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