The first part of the spell building system covers situations like finding a really cool spell that seems difficult to quantify or that you do not want to breakdown and rebuild. It is a fairly simple system to use spells from any OSR system, or retroclone OGL content. Before starting on that, here are the basic mechanics of the spell building system.

The system adds point values based on effect, range, duration, and any possible areas affected by the spell. The total is the spell's difficulty rating. Success in casting a spell is determined by rolling 1d20 + Intelligence score + Magic-Users's level to equal or exceed the magic-user's Saving Throw + difficulty rating. At the referee's discretion, a difficulty rating can be converted to a spell level for use in Vancian spellcasting. Like Cleric spells, converting a difficulty rating to a spell level is reversible.

This may seem like a mouthful, but to help with ease of calculation, I put Intelligence score + Caster Level on the character sheet. You'd think this wouldn't be a big deal, but it seems to make calculation easier. Since the Saving Throw already appears, it is just a matter of looking up the difficulty rating in the spell book.

In this post, I'll be taking OGL spells from various places and converting them to a difficulty rating.

Simple Conversion to a Difficulty Rating

Use the table below to convert spells to a difficulty rating:

†Cantrips cannot do any damage. Using cantrips assumes the use of Tim Brannan's Cantrips for Basic Level Games system.

The difficulty rating in the table above is the midpoint for a spell level. Feel free to adjust the difficulty rating up or down by up to 2 points. In other words, a second level spell can have a difficulty rating from 13 to 17.

In looking for spells to serve as good examples, I looked for OGL spells from the LInks to Wisdom section on spells.

First, let's take a look at the Dreadcube (click the link for a full description). It is listed as a 7th level spell, so the new difficulty rating for this spell would be 34. It has multiple effects, so normally, I'd adjust the rating up, however, these effects can also potentially harm the caster, so I'll leave it where it is. OGL Link for Dreadcube

For those that wonder about the odds, let's take a magic-user with an INT 13 (the minimum score required to cast a 7th level spell). We'll say that the magic-user is at 14th level. The M-U will roll 1d20 + 13 (Intellegence score) + 14 (Caster Level) to be greater than or equal to 5 (Saving Throw at 14th level) + 34 for a total of 39. The magic-user will have a 45% chance of successfully casting the Dreadcube.

That seems a bit low, but 13 is the minimum Intelligence to cast a 7th level spell. Most Magic-Users for my players have an INT of 15 or 16 at least. Just for the sake of comparison, a 14th level Magic-User with an Intelligence of 15 attempting to cast this spell has a 55% chance of success.

Here's another favorite of mine from the Space Age Sorcery pdf, Pretervolve. (click on the link to download the free version). It is listed as Level 5, so the difficulty rating is 27. Seeing as there is a permanent effect after the spell wears off, I'd add a point to make the final difficulty rating a 28.

Again for the odds, we'll say that a 9th level Magic-User with an INT of 13 will cast Pretervolve. The M-U will roll 1d20 + 13 (Intellegence score) + 9 (Caster Level) to be greater than or equal to 7 (Saving Throw at 9th level) + 28 for a total of 35. The magic-user will have a 40% chance of successfully casting the spell.

Again, many characters will have a higher INT score. A 9th level Magic-User with a 15 Intelligence score has a 50% chance of success.

Section 15 of the OGL for the Pretervolve spell is:
Space-Age Sorcery, Copyright 2013, Hereticwerks; Authors James Garrison, Eric Fabiaschi, Porky

Here's another favorite, the Auric Devourer (read the post for the full description). It is listed as 1st level, so I convert the difficulty to 11. Since the description states that it is easy to cast, I'll bump it down to 10.

I'll come back to this spell another time when going through the building system itself, because it mentions other factors that will be covered later. It has a listed casting time and an area of effect.

Really quickly, a 1st level Magic-User with a 13 Intelligence will have a 50% chance of success to cast this spell.

Lastly, let's mention the Cantrips. They are not necessary at all, but I mention them here because they add something fun. Using the table, Blackflame starts at a difficulty rating of 7, and I'd leave it at that. Looking at other cantrips in the list, I would probably make Flavor a rating of 5. The key to using cantrips is that they cannot do any damage. Regardless of difficulty rating, any spell that does damage must be at least a 1st level spell.

A 1st level Magic-User with a 15 Intelligence (to represent most player characters), will have an 85% chance to cast Flavor and a 75% chance to cast Blackflame.

Using Difficulty Ratings with Class

Okay, I can turn a spell level into a difficulty rating. Now what? How does this work?

At this point, you could use the Spell Point system I mentioned to track the ability to cast spells. The cost of casting the spell is the spell level. Casting a spell deducts from a character's Mana and when Mana is zero, no more spell for you.

For your conveience, here is the table for Mana per level.

If the referee prefers, the traditional spell slot system can also be used.

Another alternative is to say that a Magic-User can attempt to cast a spell in his spellbook until it fails. If the Magic-User has Sleep in the spellbook, he can cast it until the dice betray him. For low level magic-users, this means that they can likely cast more spells per day, but it removes the need for bookkeeping.

Next time, we'll look at building spells based on effect, range, duration, and other factors.