The Skullduggery Die

Ossuary Bone Dice Set CC-BY 2.0 Mindmax

This is an idea that was shared in the comments find my most frequent commenters and loyal reader McChuck and I decided to run with and do the number crunching for.

The credit is therefore not mine but my mission is to share the awesome.

In one of my recent Classy World Building articles, he took note of my use of a modified version of Dungeon Crawl Classics RPG's deed die. If you're not familiar with the rule, in essence, Warriors in DCC RPG may attempt to trip, blind, disarm, maim, confuse, etc. whenever they are making an attack. They describe what additional effect they would like their attack to have: as a part of every attack warriors roll a deed die which serves as their attack bonus for that attack, and is also added to the damage of the attack. If the number is three or higher, some version of the desired effect takes place. 

In Dungeon Crawl Classics RPG RAW the number on the Deed Die should indicate a scale of success based on what the warrior was attempting to achieve. A 3 might indicate, on an attempt to disarm, that the opponents grip on the sword has been interrupted, and they suffer a penalty on the attack, while it might take as high as a 5 to actually swat the weapon out of their hands, and a seven would indicate that the weapon goes flying off well beyond reach. In practice, however, most GMs give the Warrior the full effects they were looking for when they roll a three or higher on the deed die. This mechanic makes Warriors a lot more fun to play and is far simpler than the D&D3e feat system that inspired it.

For the Drowned World I'm using a derived mechanic, but I am sticking to the standard dice used in D&D. None of the funky D5s & D7d used in DCC.

McChuck asked me if it might not be simpler to use a similar die for thief abilities rather than using the pips on a d6 system I was pirating from Lamentations of the Flame Princess.

I resisted the idea at first, because it seemed hey little too reductive, but then I thought of some of the most fun I've had in tabletop games using the revised Crimson Dragon Slayer d20 rules from Venger Satanis, which suggest that thieves should normally succeed at thieving tasks unless the chances of success are slim. At which point the GM should have them roll a d20 plus their level and beat a 15 to succeed.

I found this a perfectly fun way of handling thieves in the few times I have used Crimson Dragon Slayer d20. Why wouldn't I consider further simplifying the mechanic beyond the simple d6 system I was using? What have I got to lose?

So, I grabbed my Swords & Wizardry core book and made a few simple rules for myself in order to determine which die I would use at which level:

A successful role would be a four or higher on a skullduggery die. Therefore, on a D4 there's a 25% chance of success. On a d6 there's a 50% chance. On a d8 there's a 62.5% chance of success. On a d10 a 70% chance of success, and on a d12, a 75% chance. D20 offers an 85% chance, and 2d8 a 95.3%.

• Only the standard D&D array, not the sake dice would be used for the initial table. 
• Nothing lower than a D4 would be used.
• I would look at all of the skills except climb walls and derive an average percentage chance to succeed at a thief skill
• I would allow a pretty generous 10% rounding up of that number to determine when the next die would be. So, once the average thief skill is at 40%, the Skullduggery die will go up to a d6. Once it's at 55%, I will round up to a d8 and so on.
• I will do a second set with slightly better resolution including Zocchi dice up to a d14.
• I will assume that this means that even classes are not going to be overly specialized. Thieves can use their skullduggery dice for spy missions and disguise just like an assassin can.
• I may choose one signature Thief skill for each class that gets a +2 on the die roll.

Here are my results for Thief, Assassin, and Monk characters in S&W.

Level	Thief avg.%	Thief	Assassin	Monk
1	18.5	D4		D4
2	25
3	32.2		D4
4	36.2
5	40.2	D6		D6
6	44.2
7	51.6		D6
8	60.6	D8		D8
9	68.6
10	76.6	D10	D8	D10
11	88	D12		D12
12	96	D20	D10	D20
13	100	2D8	D12	2D8
14	100		D20
15	100		2D8
16	100
17	100
18	100
19	100
20	100

Thief +2 to Climbing Walls

Assassin +1 to Disguise and Impersonation, +1 to Climb Walls

Monk +1 to Balance and Acrobatics, +1 to Climb walls

Obviously,  offering just a +1 makes a substantial difference to probability,  and so modifiers should be used,  especially at low levels, but they ought to be very carefully considered.