Riffing on 2d6: Turning Undead

Alright, time to look at another classic application of 2d6, the "Turn Undead" mechanic. Unlike reaction rolls where the range a roll falls into matters, turning the undead requires beating a target number that depends on the level of the cleric as well as (the hit dice of) the undead creature to be turned. Traditionally the numbers 7, 9, and 11 have to be beat. Here's (part of) the relevant table from OD&D (Vol 1, page 22):

Cleric Level	Skeleton (1/2 HD)	Zombie (1 HD)	Ghoul (2 HD)	Wight (3 HD)
1	7	9	11	N
2	T	7	9	11
3	T	T	7	9
4	D	T	T	7

Entries marked "N" cannot be turned at all, those marked "T" or "D" are automatically turned or destroyed. The chances of getting at least the required target number on a straight 2d6 roll are as follows:

2d6	Percentage
7	58.33%
9	27.78%
11	8.33%

If we compare cleric level against undead hit dice, this means that clerics have a fairly small chance, less than a third in fact, for turning creatures of power equal to themselves. But at least that's a nice pattern: The entire table uses 9 for creatures equal to the cleric's level in hit dice. So we may not like the low chance of success, but at least the mechanic is consistent (for the most part anyway, vampires ruin it a bit with their variable hit dice).

In Holmes D&D (page 12) and B/X D&D (page X5) we find the exact same table but with a small twist: Some undead creatures get different hit dice. Holmes keeps skeletons at 1/2 HD but changes zombies to 2 HD, B/X also changes skeletons to 1 HD. Curiously ghouls and wights remain at 2 HD and 3 HD respectively thus ruining the pattern that governed the table in OD&D.

Let's briefly look at AD&D in this regard (Dungeon Master's Guide, page 75). The hit dice are as in B/X D&D but the mechanic is now a d20 roll against a target number. Also clerics can affect undead creatures a lot more powerful than themselves:

Cleric Level	Skeleton (1 HD)	Zombie (2 HD)	Ghoul (2 HD)	Shadow (3+3 HD)	Wight (4+3 HD)	Ghast (4 HD)	Wraith (5+3 HD)	Mummy (6+3 HD)
1	10	13	16	19	20	N	N	N
2	7	10	13	16	19	20	N	N
3	4	7	10	13	16	19	20	N
4	T	T	4	7	10	13	16	20

Obviously nothing of the formerly elegant mechanic is left at this point: We have ghasts, monsters with fewer hit dice than wights that are nevertheless harder to turn, we have target numbers missing (no 19 at level 4 for instance), we suddenly jump to "T" for two monsters at level 4 (presumably because shadows got inserted into the table), etc. Gary, what were you thinking? And what about the chances of success?

1d20	Percentage
4	85%
7	70%
10	55%
13	40%
16	25%
19	10%
20	5%

So for level 1 clerics it got a little harder to turn skeletons, but turning zombies and ghouls actually became quite a bit easier. And now they even have a shot at turning shadows and wights! I guess the question is whether they should have a chance to turn undead creatures more than four times as powerful as they are? But that gets us too far off track, after all we want to study the 2d6 mechanic, not get bogged down in the thankless task of fixing AD&D.

One thing the AD&D version points to is that there probably should be more levels of difficulty. In the original 2d6 version we go from a 58% chance for turning directly to a 100% chance, a level of progress unheard of in other areas (combat or thieves' skills for example). And of course there are plenty of possible target numbers:

2d6	Percentage
2	100%
3	97.22%
4	91.67%
5	83.33%
6	72.22%
7	58.33%
8	41.67%
9	27.78%
10	16.67%
11	8.33%
12	2.78%

What sticks out, because they fit with the existing "7, 9, 11" pattern, are the target numbers "5" and "3" that could precede a "T" result thus extending the table. And that's exactly what Labyrinth Lord does (page 9):

Cleric Level	1 HD	2 HD	3 HD	4 HD	5 HD
1	7	9	11	N	N
2	5	7	9	11	N
3	3	5	7	9	11
4	T	3	5	7	9

Note also that the table no longer lists specific undead creatures, probably just as much of an improvement as the wider range of target numbers. We now get the following chances for successfully turning undead:

2d6	Percentage
3	97.22%
5	83.33%
7	58.33%
9	27.78%
11	8.33%

Some may complain that the additional target numbers "nerf" the cleric too much, but I disagree. Modifying the table this way brings it closer to the more gradual progressions exhibited by other mechanics such as combat and thieves' skills. It's also fun to roll knowing that one is 97% likely to succeed, dare I say more fun than being 100% likely without a roll? What it doesn't do, however, is usefully approximate the AD&D chances for turning:

2d6	Percentage	d20	Percentage
3	97.22%	-	-
5	83.33%	4	85%
7	58.33%	10	55%
9	27.78%	16	25%
11	8.33%	20	5%

There are lots of gaps (for 2, 7, 13, and 19 on d20) that make mapping the AD&D table back to the 2d6 table too much of a hassle. Of course I am not even sure why we'd want to do that, but I had to point it out anyway because it was possible after all to get a decent approximation in the case of reaction rolls.

Yet another wonderful variation on the 2d6 table comes from Dyson Logos. He uses the notation "x/y" to indicate that on "x" or higher the undead are turned but on "y" or higher they are destroyed instead:

Cleric Level	1 HD	2 HD	3 HD	4 HD	5 HD
1	9	11	N	N	N
2	7	9	11	N	N
3	5/11	7	9	11	N
4	3/9	5/11	7	9	11
5	T/7	3/9	5/11	7	9

In this way, not only are the chances for turning "smoothed out" compared to B/X, but the same is true for the chances of destroying the undead. Note that unlike Labyrinth Lord itself, Dyson consistently assigns 9 as a target number for undead creatures with the same number of hit dice as the cleric has levels. This is actually what OD&D did (see above) but the "B/X shift" for skeletons and zombies makes it seem like clerics in his campaign have it harder than clerics in regular Labyrinth Lord or B/X games. Fixing the table to bring it in line with those percentages is easy enough though:

Cleric Level	1 HD	2 HD	3 HD	4 HD	5 HD
1	7	9	11	N	N
2	5/11	7	9	11	N
3	3/9	5/11	7	9	11
4	T/7	3/9	5/11	7	9
5	T/5	T/7	3/9	5/11	7

And there you have it, the 2d6-driven table for turning or destroying undead creatures that I would most likely use in the future. But now it seems like I spent a whole lot of time specifically on turning undead and not on 2d6. Or did I? What do we have "encoded" in that last table? It's a task that depending on the level of a character has different chances of succeeding. And actually, it can succeed "normally" and "perfectly" if we include turning and destroying undead creatures. Does that smell a little bit like a possible skill mechanic? :-)