Friday, July 31, 2020

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.



2 comments:

  1. For insanity-rules, I intend to introduce a "Humanity"-pool represented by an Usage Dice.
    See bad shit? roll humanity...
    Do BAD shit? reduce humanity by one step...
    Cheat death? reduce humanity by one step...
    and so on, and so on...

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    Replies
    1. Good idea!
      I think that the Usage Dice works best for things that are otherwise not quantifiable. Tracking arrows or bullets with usage dice is weird. Tracking "humanity" - that makes sense to me!

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