I've been on a huge grimoire kick lately. Real grimoires, that is, like the Clavicula Salomonis (Key of Solomon) and others. Not originals, of course, but the many commonly available translated editions sold by booksellers and free online (see links throughout this post). Most of these (in)famous tomes first started circulating in the late medieval/early rennaissance period, despite their claims of ancient providence, and offer a fantastic insight into what constituted "real" wizardry in the Western world during the time periods that most fantasy RPG games are based on.
There's some wild and crazy stuff to be found here. The Grimorium Verum will tell you how to invoke the demons Mersilde, who "has the power to transport anyone in an instant, anywhere" and Frimost, who "has power over women and girls, and will help you obtain their use." Plus spells for curing rabies, turning invisible (good luck getting your hands on the human skull), obtaining gold and silver, and more.
The Goetia portion of the Lemegeton even specifies the proper time to summon and bind each demon by rank ("The chife [chief] kings may be bound from 9 to 12 of ye Clock at noone & from 3  till sunset. Marquizes may be bound from 3 of ye Clock in ye after Noon till nine at night and from 9 at nt [night] till sunrising. Dukes may be bound from sunrising till Noonday in clear weather.").
Studying these works and other similar ones has given me quite a few pointers that I can see influencing future fantasy gaming sessions:
1. Magic can be ruthlessly low-down and practical. Perhaps the strongest recurring themes among the spells and rituals in these books are the garnering of sex, temporal power, and cold, hard cash.
2. Magic should really evocative. "Cure Light Wounds" is one thing, but using Key as an example, we can see offhand some most excellent chapter headings:
"Of the Blood of the Bat, Pigeon, And Other Animals."
"How Operations of Mockery, Invisibility, And Deceit Should Be Prepared."
"How To Make the Magic Carpet Proper For Interrogating the Intelligences, So As To Obtain An Answer Regarding Whatsoever Matter One May Wish To Learn."
And then there the spells themselves. Here's an incantation from the section entitled "Regarding Experiments To Be Made Regarding Hatred And Discord":
"Where are ye, SOMNIATOR, VSOR, DILAPIDATOR, TENTATOR, DIVORATOR, CONCISOR, SEDUCTOR, ye who sow discord, where are you? Ye who infuse hatred and propagate enmities,
I conjure you by him who hath created you for this ministry, to fulfill this work, in order that whenever N. shall eat of like things, or shall touch them, in whatsoever manner, never shall he go in peace."
Sure, it's almost certainly a bunch of superstitious hokum, but is it ever cool-sounding!
3. Magic can be really complicated. Some versions of D&D, particularly AD&D, already have certain spells with lengthy casting times, but these books really emphasize how much work being a wizard can be, with rituals that involve specially consecrated knives, swords, robes, parchment, water, seals, amulets, pentacles, etc. Sometimes all of the above and more for a single working! And that's even before you get past the generic stuff to things like exact astrological specifications and the need for a hare slain on the 25th of June or what have you.
4. Finally, they all have awesome magic seals that you'd be crazy to not want to rip off for your games if possible. For example:
So if you have the time, I definitely recommend you check these suckers out. Besides, if you don't, I might just have to whip out my virgin parchment, blessed stone and bat blood so I can call on Frutimiere to make you strip nude and "dance increasingly until death...with grimaces and contortions which will cause more pity than desire." And none of us want that. I hope.
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