Mythos Spells to OSR: quick & dirty conversion rules

Over the years, Call of Cthulhu has amassed a great amount of spells and rituals. The 7th ed. Grand Grimoire lists over 550! Such a horrific source of cosmic grandeur cannot go untapped... So, here are some quick & dirty untested rules for converting CoC spells to OSR spell levels. Of course, case-by-case conversion and fine-tuning is better, but with these rules you can get a rough approximation.

Magic point cost	Spell level
1-5	1
6-9	2
10-12	3
13-15	4
16-19	5
20+	6

For variable costs (e.g. 1d10), assume maximum roll.

Sanity cost	Spell level increase	Save penalty
1-1d5	+0	-0
1d6-1d10	+1	-2
1d12-1d20	+2	-4

Optional: OSR “Sanity check”

When casting a spell that comes with Sanity loss, save against Magic (with the penalty in the third column). On failure, gain a random “insanity” that lasts 1d8 days. If the margin of failure is more than 5, the effect is permanent instead.

The 3.5e/d20 SRD has some examples (scroll down to tables 6-9 and 6-10).

Or just treat this as risky casting/miscast.

POW cost	Spell level increase
0-9	+0
10-19	+1
20+	+2

Optional: OSR stat drain

Map POW onto a stat (Wisdom, Intelligence, Charisma? Maybe let the Magic-User pick each time?), and divide the cost by 5.

Optional: Sanity+POW
If the spell has both a Sanity and a POW cost, but individually they don't raise the spell level, increase the spell level by 1.

Create Myst of R'lyeh (2 magic points) = level 1.
Knot Flesh (2d6 magic points, 1d8 sanity) = level 4.
Reincarnate (10 magic points, 10 POW, 1d6 sanity) = level 5.
Soul Trap (6 magic points, 5 POW, 1d4 sanity) = level 2 (or level 3).
Wave of Oblivion (30 magic points, 1d8 sanity) = level 7.

[Monster] The Pale Menace!

More random monster generation! This time using the article "Random Monsters" by Paul Montgomery Crabaugh, which appeared in The Dragon #10 (link to the annarchive pdf). So already in 1977 the problem of jaded players knowing all the counters to all the monsters in the book existed, and Crabaugh wanted to spice things up with unexpected creatures. The generator is a simple affair, but perhaps it set the framework for latter sets of tables.

The Pale Menace

Intelligence: Highly intelligent

Alignment: Neutrality

HD: 6+1

AC 4 [LotFP AC: 17]

Move: 12"

Attack: claw for 1-10 or bite 1-10 and 1-6 poison

Special attack: Magic Missile as Magic-User 4, once per day

Special defense: regeneration, 1-4 hits per turn

Slim and gaunt, pale and wrinkled, like a hairless bear, but with the fangs of a saber-tooth tiger. Its four eyes are located equidistantly around its head, like a band of glowing red jewels (once per day, it can shoot 4 laser beams; treat as Magic Missile). 

The Pale Menace is a lesser servitor to Ithaqua. Its body irradiates coldness. If damaged, its wounds quickly get covered in hoarfrost and heal (regenerates 1-4 hits per turn). 

Lame art by me

My original rolls:

Intelligence: Highly intelligent

Alignment: Neutrality

Type: Mammal

Speed: 12”

AC: 4 [LotFP AC: 17]

HD: Dungeon level +3 (Mod: +1) [I gave this creature 6 HD]

Specials: 3

                Bite is poisonous 1-6 damage

                Regenerates 1-4 hits per turn

                Has Magic Missile

Damage: 1-10

Characteristics:

                4 eyes

                Unusually long fangs

Size: Medium (3-12’)

Limbs: 2 legs, 2 arms

Cover: Skin

Coloring: White