I’ve always had a love-hate relationship with thieves. On the one hand, they’re so damned cool, on the other hand, the skills…I’ve seen lots of different variations on how to handle thief skills. The one I like the least is the percentage-based found in OD&D, AD&D and the various clones. It seems so clunky, and has the base chance of most skills abysmally low (10% base chance of hiding in shadows? Might as well fall on your sword…). Delving Deeper assigns the same perfectly reasonable base chance to all thief skills, but they don’t ever increase with the thief level. AS&SH uses a d12 range for each skill, which I like better, but it still suffers from the AD&D problem of having a poor chance of success at low levels. With this in mind, I thought that maybe it would be better to limit the number of skills at first level, but allow a better chance of success with each. Then the player can choose what additional skills they want at each subsequent level. This has the advantage of allowing players to mold their characters as they see fit, while still having a reasonable chance that they can actually succeed at their chosen skills. What’s more, at each level the player can choose to add a bonus to a skill of their choice. The result is the rogue (I know, I know, but how many cool names are there for a thief?), which I wrote for Swords & Wizardry White Box, although it could be easily be ported to other old-school systems.

Edit to add:  Peter Fröhlich had this very idea last year.

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The Rogue – A Thief Variant for Swords & Wizardry Whitebox

The rogue is an urban thief who specializes in a mix of skills of their own choosing. Rogues use cleric attack tables, and are limited to daggers, short swords, slings and bows as weapons. They can wear leather armor but may not use a shield. Humans and Halflings can be rogues, both with unlimited advancement. Rogues get no XP bonuses for high ability scores, and must be neutral or chaotic in alignment.

Rogue Advancement

The following is the list of skills available to rogues. At first level, rogues get their choice of 1d3 skills, then gain one additional skill with each level. The base chance of success with each skill is any roll of 6 or lower on 2d6. Where an ability appears in parentheses after the skill name, that means any bonus the rogue possesses due to the named ability score applies to the skill success. If more than one ability is listed (as in STR+DEX), simply add bonuses for each listed ability. At each successive level, the rogue adds one skill of their choice, and also adds the given bonuses from the above table to any existing skill or skills of their choice (but not to the skill just chosen).

• Backstab (STR)
• Climb walls (DEX)
• Defensive fighting (STR+DEX)
• Disarming (STR+DEX)
• Find, remove or set traps (DEX)
• Find secret doors (WIS)
• Hide in shadows (DEX)
• Move silently (DEX)
• Pick locks (DEX)
• Poison admixtures (INT)
• Read secret writing & scrolls (INT)
• Sleight of hand, pick pockets (DEX)
• The art of falling (DEX)
• Tracking (WIS)

For example, Rufus the 1st level rogue gets a 2 on a 1d3 roll and so chooses two skills – backstab and pick locks. His base chance of success is 2-6 in 2d6 with both. Assuming he has a 15 DEX, which gives him a +1 bonus, his chance of success at picking locks rises to 2-7 in 2d6. At second level, Rufus chooses to add hiding in shadows, and further chooses to add his available +2 bonus to the backstab skill, giving him a 2-8 in 2d6 chance of success. Note that Rufus could also have chosen to split the available +2 bonus between backstab and picking locks. In no case may any chance of success rise to higher than 11 in 2d6.

Backstab: If a rogue is able to maneuver behind an opponent, he can opt to backstab with a dagger. A normal to-hit roll is made at +4, and if it is successful, the rogue makes a skill check for double damage. Failure of the skill check means normal damage is done. At 4th level, this rises to triple damage, and at 8th level quadruple damage. Magical bonuses are added before the damage factor is calculated. Note that victims must have a back in order to be backstabbed, so creatures like slimes or oozes are not affected by backstabs.

Climb Walls: The rogue can climb sheer surfaces that would normally be unclimbable by others. Assume any failure occurs at halfway up the surface being climbed, and results in a fall. Falling damage is normally 1d6 per 10 feet fallen (but see The Art of Falling, below).

Defensive Fighting: This allows the rogue to parry an attack so that his opponents get a to-hit penalty of -1 plus any DEX or STR bonuses. This gives a range of -1 to -3. This penalty is in effect in a given combat round as long as the rogue forfeits their attack that round. However, a successful skill check allows the rogue to also attack normally that round.

Disarming: The rogue is adept at causing opponents to drop any one held weapon on a successful skill check roll. The rogue must have initiative to attempt to disarm his opponent, and cannot attack in the same round he chooses to disarm. A failure means his opponent can attack normally.

Find, Remove or Set Traps: With appropriate tools, rogues can try to detect and then deactivate traps. Note these are separate skill checks, and failure to deactivate a trap once detected means it has been sprung. Once a trap is disarmed, a rogue can attempt to re-set the trap with a third skill roll.

Find Secret Doors: Rogues will notice secret, hidden or otherwise concealed doors with a successful skill check. This is in addition to any normal check for finding secret doors, which is 1-2 in d6 for humans. If that initial check fails, the rogue would then gain an additional skilled attempt at his base 2-6 in 2d6.

Hide in Shadows: A Rogue can hide unseen by using shadows or other concealment to his advantage. He can can then move at half-normal movement rate, as long as there are shadows present to hide in. While moving in such a fashion, the rogue must also check his move silently skill, or risk alerting others to his presence. As an example, our 2nd level rogue Rufus is in a brightly-lit castle hallway with guards posted at the far end. He cannot use his hide in shadows skill to avoid being seen by the guards, as there are no shadows or other forms of concealment available. If, however, the hallway is lined with large tapestries and statues, he could attempt to hide in one place. If he wanted to move down the hallway without being detected, he would have to make two skill checks, one for hiding in shadows, and one for moving silently. Note that a failure of this skill may not be known to the rogue – he may think he is hiding, when in fact he is visible.

Move Silently: A rogue can use this skill to move at half his normal move rate without making any discernible noise. Similar to hiding in shadows, failure of this skill is not always known – he may think he is moving without sound, when in fact he is heard by others.

Pick Locks: With appropriate tools, a rogue can attempt to pick any non-magical lock mechanism.

Poison Admixtures: Rogues with this skill are adept at mixing poisons with various effects which can be mixed into drink as a liquid or spread as a sticky, fast-drying form on weapon blades. At levels 1-4, rogues can mix poisons that cause drowsiness or confusion. At levels 5-8, rogues can additionally mix sleep or sickness poisons, and at levels 9 and above, rogues can mix poisons that kill. Any poison takes effect within 1d3 turns after being ingested (liquid form), or 1d3 rounds after being introduced to the bloodstream via a weapon wound (which implies a successful attack roll). The skill check indicates potency – a failure indicates the poison has the next lower-level of effect on the poison table, as if the victim made a saving throw automatically – although they are still entitled to an actual save in any case. For 1st-4th level rogues, failure means the poison has no effect, but they will not know that until they try to use it. Poisons spread on blades survive but one successful attack, whether they have any effect or not.

As an example, Rufus is now 4th level and chooses the poison admixture skill. He gathers the required ingredients from a local apothecary at a cost of 150GP, and spreads the poison so made on his dagger blade. The referee makes a skill check in secret, which is 9 on 2d6. Rufus’ poison is completely ineffective, but he does not know it. At 5th level, Rufus can attempt to make a sickness poison, but failure of his skill check means it has only the confusion effect when it is actually delivered to the victim, who, if he makes his saving throw, would thus not feel any effect.

Read Secret Writing & Scrolls: Success with this skill allows the rogue to decipher and infer the meaning  of  any non-magical writing such as that found on treasure maps, in secret correspondence, or in other coded messages. At 8th level and above, rogues can read magic-user scrolls, but a roll of 12 means the spell is reversed (or fails if the effect cannot be logically reversed).

Sleight of hand, pick pockets: Rogues with this skill are adept at stealing or secreting small objects about a victim’s person. Failure of the skill means the intended target is aware that the rogue is trying to steal or place an item from or on their person.