GURPS Magic: How to Make a Multi-Use Trap Out of Nearly Any Spell.

Someone in the GURPS forums is having trouble getting GURPS Magic as written to make magical traps that are more than one-shots. Here's one way:

Just like "All Charged Up over Magic Items" (Pyramid #3/36: Dungeon Fantasy) Spell Stone (GURPS Magic p. 60) is the solution. In this case you'll also want Link (Magic p. 131) to activate the spell. As with "All Charged Up Over Magic Items" if you can assume that one item can have multiple "virtual" spell stones, then you'll only need one Link enchantment to control them all.

Follow the steps in "All Charged Up over Magic Items" and then:
7. Decide on the triggering conditions. This generally needs to be something a normal person can perceive (e. g. someone crosses the threshold) but can include magical senses if the appropriate spells are also enchanted.
8. Add 200 Energy (and therefore $5000 in Dungeon Fantasy) for the Link plus the cost for any spells (Watchdog, Sense Magic etc.) that are needed for the triggering condition (if any).

Example: Christoff the Grouchy wants a Fire Cloud (Magic p. 75) trap doing 2d damage over a 3 hex area if anybody comes within 1 yard of it with 10 charges. This has a normal cost of 6 energy. Making it into spell stones is 120 Energy per charge (and costs $120 using the economic assumptions in the article). It also requires a gem (or materials or whatever -although gems are pretty classic for magical traps) that costs $600 per charge. On top of all of this you'll need Link enchanted for 200 energy ($5000). The trap therefore costs $12,200. It might be worth the thief risking getting burned a bit in order to steal it instead of just shooting it...