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 Designing A Trap

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Kilsek
Encounter Killer
Defender of the Faith: DM

Encounter KillerDefender of the Faith:  DM
Kilsek

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PostSubject: Designing A Trap   Fri Aug 22, 2008 10:07 am

Mechanical Traps

Simply select the elements you want the trap to have and add up the adjustments to the trap’s Challenge Rating that those elements require (see Table: CR Modifiers for Mechanical Traps) to arrive at the trap’s 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 don’t 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 requires—or, 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.
Table: CR Modifiers for Magic Traps Feature CR Modifier

1. See the note following Table: CR Modifiers for Mechanical Traps.

Highest-level spell + Spell level OR +1 per 7 points
of average damage per round1
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 CR—either 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.
Table: CR Modifiers for Mechanical Traps Feature CR Modifier

1. Rounded to the nearest multiple of 7 (round up for an average that lies exactly between two numbers).

Search DC
15 or lower -1
25-29 +1
30 or higher +2
Disable Device DC
15 or lower -1
25-29 +1
30 or higher +2
Reflex Save DC (Pit or Other Save-Dependent Trap)
15 or lower -1
16-24 —
25-29 +1
30 or higher +2
Attack Bonus (Melee or Ranged Attack Trap)
+0 or lower -2
+1 to +5 -1
+6 to +14 —
+15 to +19 +1
+20 to +24 +2
Damage/Effect
Average damage +1/7 points1
Feature CR Modifier
Miscellaneous Features
Alchemical device Level of spell mimicked
Liquid +5
Multiple target +1 (or 0 if never miss)
Onset delay 1 round +3
Onset delay 2 rounds +2
Onset delay 3 rounds +1
Onset delay 4+ rounds -1
Poison
Black adder venom +1
Black lotus extract +8
Bloodroot +1
Blue whinnis +1
Burnt othur fumes +6
Deathblade +5
Dragon bile +6
Giant wasp poison +3
Greenblood oil +1
Insanity mist +4
Large scorpion venom +3
Malyss root paste +3
Medium spider venom +2
Nitharit +4
Purple worm poison +4
Sassone leaf residue +3
Shadow essence +3
Small centipede poison +1
Terinav root +5
Ungol dust +3
Wyvern poison +5
Pit spikes +1
Touch attack +1
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 you’ve added to the trap to get the modified base cost.

The final cost is equal to (modified base cost × Challenge Rating) + extra costs. The minimum cost for a mechanical trap is (CR × 100) gp.

After you’ve 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).
Table: Cost Modifiers for Mechanical Traps Feature Cost Modifier

1. Multiply cost by 20 if trap features automatic reset.

Trigger Type
Location —
Proximity +1,000 gp
Touch —
Touch (attached) -100 gp
Timed +1,000 gp
Reset Type
No reset -500 gp
Repair -200 gp
Manual —
Automatic +500 gp (or 0 if trap has timed trigger)
Bypass Type
Lock +100 gp (Open Lock DC 30)
Hidden switch +200 gp (Search DC 25)
Hidden lock +300 gp (Open Lock DC 30, Search DC 25)
Search DC
19 or lower -100 gp × (20 - DC)
20 —
21 or higher +200 gp × (DC - 20)
Disable Device DC
19 or lower -100 gp × (20 - DC)
20 —
21 or higher +200 gp × (DC - 20)
Reflex Save DC (Pit or Other Save-Dependent Trap)
19 or lower -100 gp × (20 - DC)
20 —
21 or higher +300 gp × (DC - 20)
Attack Bonus (Melee or Ranged Attack Trap)
+9 or lower -100 gp × (10 - bonus)
+10 —
+11 or higher +200 gp × (bonus - 10)
Damage Bonus
High strength rating (ranged attack trap) +100 gp × bonus (max +4)
High Strength bonus (melee attack trap) +100 gp × bonus (max +Cool
Miscellaneous Features
Never miss +1,000 gp
Poison Cost of poison1
Alchemical item Cost of item1
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.
Table: Cost Modifiers for Magic Device Traps Feature Cost Modifier
Alarm spell used in trigger —
One-Shot Trap
Each spell used in trap +50 gp × caster level × spell level, +4 XP × caster level × spell level
Material components + Cost of all material components
XP components + Total of XP components × 5 gp
Automatic Reset Trap
Each spell used in trap +500 gp × caster level × spell level, +40 XP × caster level × spell level
Material components + Cost of all material components × 100 gp
XP components + Total of XP components × 500 gp
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.
Trap CR Base Craft
(Trapmaking) DC
1-3 20
4-6 25
7-10 30
Additional Components Modifier to Craft
(Trapmaking) DC
Proximity trigger +5
Automatic reset +5
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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