Mechanical Traps: Simply select the elements you want the trap to have and add up the adjustments to the traps Challenge Rating that those elements require (see Table: CR Modifiers for Mechanical Traps) to arrive at the traps final CR. From the CR you can derive the DC of the Craft (trapmaking) checks a character must make to construct the trap.
Magic Traps: As with mechanical traps, you dont have to do anything other than decide what elements you want and then determine the CR of the resulting trap (see Table: CR Modifiers for Magic Traps). If a player character wants to design and construct a magic trap, he must have the Craft Wondrous Item feat. In addition, he must be able to cast the spell or spells that the trap requiresor, failing that, he must be able to hire an NPC to cast the spells for him.
Challenge Rating of a Trap
To calculate the Challenge Rating of a trap, add all the CR modifiers (see the tables below) to the base CR for the trap type.
Mechanical Trap: The base CR for a mechanical trap is 0. If your final CR is 0 or lower, add features until you get a CR of 1 or higher.
Magic Trap: For a spell trap or magic device trap, the base CR is 1. The highest-level spell used modifies the CR (see Table: CR Modifiers for Magic Traps).
Average Damage: If a trap (either mechanical or magic) does hit point damage, calculate the average damage for a successful hit and round that value to the nearest multiple of 7. Use this value to adjust the Challenge Rating of the trap, as indicated on the tables below. Damage from poisons and pit spikes does not count toward this value, but damage from a high strength rating and extra damage from multiple attacks does.
For a magic trap, only one modifier applies to the CReither the level of the highest-level spell used in the trap, or the average damage figure, whichever is larger.
Multiple Traps: If a trap is really two or more connected traps that affect approximately the same area, determine the CR of each one separately.
Multiple Dependent Traps: If one trap depends on the success of the other (that is, you can avoid the second trap altogether by not falling victim to the first), they must be treated as separate traps.
Multiple Independent Traps: If two or more traps act independently (that is, none depends on the success of another to activate), use their CRs to determine their combined Encounter Level as though they were monsters. The resulting Encounter Level is the CR for the combined traps.
Mechanical Trap Cost
The base cost of a mechanical trap is 1,000 gp. Apply all the modifiers from Table: Cost Modifiers for Mechanical Traps for the various features youve added to the trap to get the modified base cost.
The final cost is equal to (modified base cost x Challenge Rating) + extra costs. The minimum cost for a mechanical trap is (CR x 100) gp.
After youve multiplied the modified base cost by the Challenge Rating, add the price of any alchemical items or poison you incorporated into the trap. If the trap uses one of these elements and has an automatic reset, multiply the poison or alchemical item cost by 20 to provide an adequate supply of doses.
Multiple Traps: If a trap is really two or more connected traps, determine the final cost of each separately, then add those values together. This holds for both multiple dependent and multiple independent traps (see the previous section).
Magic Device Trap Cost
Building a magic device trap involves the expenditure of experience points as well as gold pieces, and requires the services of a spellcaster. Table: Cost Modifiers for Magic Device Traps summarizes the cost information for magic device traps. If the trap uses more than one spell (for instance, a sound or visual trigger spell in addition to the main spell effect), the builder must pay for them all (except alarm, which is free unless it must be cast by an NPC; see below).
The costs derived from Table: Cost Modifiers for Magic Device Traps assume that the builder is casting the necessary spells himself (or perhaps some other PC is providing the spells for free). If an NPC spellcaster must be hired to cast them those costs must be factored in as well.
A magic device trap takes one day to construct per 500 gp of its cost.
Spell Trap Cost
A spell trap has a cost only if the builder must hire an NPC spellcaster to cast it.
Craft DCs for Mechanical Traps
Once you know the Challenge Rating of a trap determine the Craft (trapmaking) DC by referring to the table and the modifiers given below.
Making the Checks: To determine how much progress a character makes on building a trap each week, that character makes a Craft (trapmaking) check. See the Craft skill description for details on Craft checks and the circumstances that can affect them.
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