# How can I tell if an adventure will work with Scarlet Heroes?

The Scarlet Heroes Quickstart provides "an overlay for your favourite old-school game to make its adventures playable for single PCs," which sounds pretty great (I'm interested in twosies, but turned off by the Red Tide setting). What systems are meant by "old-school" in this context? Is the overlay feature specific to adventures from particular D&D editions, or are some other old-school games also usable? What about OSR modules?

How can I tell if adventures from a particular system will work with Scarlet Heroes' twosies overlay?

In its fundamentals, Scarlet Heroes works almost identically to D&D editions prior to 3rd, and so any adventure that is designed for one of those editions or an OSR game designed to mimick one of those editions will be playable.

## Armour Class

The one caveat is that a few OSR games use ascending armour class (AC), which must be converted to the pre-3e standard of descending AC used by Scarlet Heroes. Fortunately this is a simple 1:1 conversion that can be done on the fly or marked in the adventure's notes prior. The exact conversion depends on what the “unarmoured” AC in a particular system is, relative to Scarlet Heroes' unarmoured descending AC of 9. Fortunately, there are only a few ascending AC variations:

• If the unarmoured ascending AC of an adventure's native game is 10, find the Scarlet Heroes equivalent by taking 19 and subtracting the ascending AC from it. e.g. a monster's ascending AC 16 would be 19 − 16 = 3 and convert to descending AC 3; AC 21 would convert to AC −2 (yes, negative AC is a thing).

• If the unarmoured ascending AC of an adventure's native game is AC 0 (e.g., Adventurer Conqueror King System), then convert by subtracting the ascending AC from 9 to get the descending AC. e.g., a creature with AC 2 would have descending AC 7; a creature with AC 11 would have descending AC − 2.

In general, the number to subtract an ascending AC from to get the descending AC that Scarlet Heroes expects can be found by adding the unarmoured ascending AC to the unarmoured descending AC (e.g., for most ascending AC systems which have an AC 10 base, you would add 10 + 9 = 19; for ACKS you would add 0 + 9 = 9).

If the system uses basically the same scale of numbers for hit dice and damage dice as pre-third-edition D&D, it should work fine with SH's system. Thus, just about any game that advertises itself as being "OSR" should be workable.

The key to this is the damage scaling system. So long as the monsters have about the same sort of hit dice as they do in pre-3e D&D, the scaling works. Thus, a few points of difference in armor class, or different damage dice, or slight hit die differences don't really matter. You'll run into problems if you try to run 3e+ adventures with it because the latter editions tend to allow much higher hit dice totals than the early ones.