D20 Patch: Basics

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Basic Rules & Changes
d20 Patch uses Dungeons & Dragons 3e and the Pathfinder 3rd-party content that already replaces much of the base system. In the Pathfinder Core Rulebook, before anything else, the first two pages contain these testimonies from the design team: "That shelf you have full of great adventures and sourcebooks (many of them very likely from Paizo)? You can still use everything on it with the Pathfinder RPG. In fact, that was what convinced me to come on board the Pathfinder RPG ship. I didn’t want to see all the great stuff that had been produced thus far swept under the rug." "The Pathfinder Roleplaying Game did not start out as a standalone game. … So while the Pathfinder RPG is compatible with the 3.5 rules, it can be used without any other books." Pathfinder, by word of the people who made it, was intended for use with D&D 3.5 content. Meanwhile, 3.5 states on page 6 of the Monster Manual: "This is an upgrade of the d20 System, not a new edition of the game. This revision is compatible with existing products, and these products can be used with the revision with only minor adjustments." Pathfinder includes 3.5, and 3.5 includes the d20 system. If your group votes to houserule otherwise, that's fine; personally, I find it a hassle to convert prices from d20 Modern.

These are some changes to the core system that are referenced often in d20 Patch.

Players Roll
Whenever there is a d20 roll against another character, the PC rolls their stat and the NPC takes 10. Players roll their spells and compare against the save DC of the target, rolling once per target. If a PC is rolling against another PC, the player acting last rolls (typically the one trying to dodge an attack).

Any time the act of rolling or rolling high and failing anyway would give a non-GM player information their character wouldn't know, the GM rolls instead. Examples include rolling perception against the stealth of a creature they don't know about, rolling attack against a square they think an invisible creature is in, etc. Determining the difficulty of a task (the skill DC, or AC of a target) through trial and error is information the character would know.

Advantage/Disadvantage
For each advantage and disadvantage, roll an extra die of the same type (if an effect adds another type of die to the roll, that type is unaffected). For each advantage, ignore the lowest die result. For each disadvantage, ignore the highest. When making multiple rolls together (such as with Multiweapon Fighting), advantages and disadvantages apply to the total number of dice.

When rolling against a DC with advantage/disadvantage, the roll applies the opposite. For example, a disadvantage to a creature's AC grants an advantage to attack rolls against them.

Aiming
Area Targeting: Area effects now require an attack roll to correctly place the center of their effect (squares typically have AC 5). For example, Fireball can no longer be aimed flawlessly through an arrow slit to hit everyone inside; the arrow slit gives the target destination a +8 cover bonus (AC 13).

Breaking Cover: If a target is destroyed or killed, halve any cover bonus the target provides to other targets (maximum +5). For example, if a Fireball blasts through a wall, targets on the other side get a +5 cover bonus instead of total cover.

Sweep: A sweep is a new shape of area effect. To target one, draw a line of effect from the center of your space to the edge of the effect's range. Then, pivot that line in a circle centered on you. All possible targets in that circle are targeted in the order the line passes the center of their space, and provide cover to later targets. Anchored targets block line of effect for all future targets. For example, a sweep effect could start in front of you, move upwards in a circle around you, then stop at the floor behind you (unless the floor is destroyed).

Rolls
Critical Range: This is the number of results that are a critical threat. For a critical range of 2, rolling a natural 19 or 20 against AC is a critical threat, while rolling a natural 1 or 2 against the attack DC is a critical threat.

AC and Reflex: All cover and dodge bonuses apply equally to both AC and reflex saves. Shield bonuses are now cover bonuses instead. // Future note: AC is abandoned entirely in favor of reflex saves. Armor will give a combination of a reduced cover bonus and a fraction of its hardness. These changes make the system a lot simpler, with fewer numbers to track and more intuitive mechanics for things like flat-footed and armor's maximum dexterity. "AC" will start lower for most armor but scale, with a rogue's reflex keeping pace with a wizard's BAB, making AC more relevant at higher levels but still fall off.

Fumbles: There is no automatic failure on a natural 1. Ever.

Glossary
Heighten: To heighten a spell is to cast it using a higher-level spell slot. It is treated as a spell of the slot's level level for all purposes. As examples, "Heighten 3rd" means to cast it as a 3rd-level spell, while "Heighten+1" means to cast it as a spell 1 level higher. The former always applies first and cannot be used with other heightening of the same kind, while the latter applies second and can.

Pass/fail: I use "pass" instead of "success". For example, "A successful check allows you to X. For every 5 points by which your check result exceeds the DC, Y." (d20pfsrd copypasta) becomes "If you pass, X. For every 5 you pass by, Y."

Reaction: Immediate action.

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