At my table I do this thing where players can pretty much ask me to use any spell from any source and, after some deliberation and twiddling, I let them.
Enters the Grand Grimoire of Cthulhu Mythos. The 7e CoC magic rules are interesting but do not map directly on Wonder & Wickedness, and I want a way to handle them directly with not much problems.
The porting of course implies that your game is old school, with its specific ways. So a lot of premises are different, and we need different outcomes. The approach is as it follows:
Do not use sanity costs. Everyone knows magic is real, and that faeries do in fact live in the hedges between the oat fields and the orchard.
Each spell uses only slot, or one mana point. If a spell allows overspending, each extra slot or mana point accounts for 5 MP, and they can come from ambrosia, mana tar, magic vessels or power stones. When mana runs out, take 1 damage for each MP the spell costs. So, yes, you can cast a lot.
POW costs reduce wisdom: each 5 whole points reduces wisdom by 1 for a month. Leftovers are ignored at the end of the adventure. Likewise, use the adjusted wisdom + unspent spells or mana when you need to roll a contest using POW, and the victim can instead elect to make a roll using their save instead of wisdom. In Mageblade! use wisdom plus the Caster focus instead of using the unspent mana.
Mageblade! casters have two ways: they can either learn the Jevnacack Praxis and cast as above, or they must use overcasting. This means that if they pass all the overcasting rolls the spell does not cost that first point of mana, and if they fail the rolls they can spend one mana point to reroll any roll. And of course they can run out of mana while doing that, which makes for some terrible news, as they either take damage from the cost of the spell or suck up the consequences of the failed overcasting rolls.
I also recommend letting anyone cast those spells, even if they have no mana. The first time a spell is cast by a non-caster, make an intelligence roll to see if it’s been learnt. Then use the overcasting rules as above, but the damage in hit point is paid in advance, and if one of the rolls fails the cost is either paid again or the spell fails.
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