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.