[RPG] How to set up a long-range alarm for the fortress

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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.

Best Answer

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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