Eldritch Might, for want of a better term, is how Monte Cook published his new magical Dungeons & Dragons 3rd Edition ideas through his own imprint, Malhavoc Press. As with other Malhavoc, White Wolf distributed these through its "Sword and Sorcery Studios" under the Open Gaming License.
This series presents its lore as excerpts from the wizard Malhavoc's library. (Compare Thralhavoc from Hellbound: The Blood War.) That library supposedly contained the full Book Of Eldritch Might, a sentient tome which the second of this series will stat out.
Monte did the first 48-page Book of Eldritch Might in 2001. The sequel, Songs and Souls of Power, came out the following year at 72 pages. The "final" one was The Nexus the year after that: 96 pages, such that it had to be bound like a trade paperback. Some years later this trilogy got edited, rebound together, and republished in a single book.
The Book of Eldritch Might
This starts with Feats.
The next chapter is Prestige Classes: the emo Embermage who cuts himself for flame, the tattooed barista Graven One beta-testing Arcana Unearthed's Rune Lord, and the Mirror Master. Why self-mutilation was such a theme in this book, we won't ask, but one question we won't need to ask is why Wizards of the Coast didn't want any of this stuff. Then come over a dozen pages of spells.
The next twelve pages are for magical items. We'll be seeing the Doomskull and the Star of Blood in the Banewarrens - both as banes. Notable here is the Staff Of Eldritch Might which is your Staff of the Magi but even more ridiculously overpowered. At least it's marked as Artifact.
Magic constructs close out this short book.
Songs and Souls of Power
Here Monte sets out his bard and his sorcerer, against Wizards', interposing them at the very start of the book. This vision of the bard is a sonic sorcerer itself, well beyond D&D canon up to then, explaining why WotC said nuh-uh. There's also a goddess, Jode. After some relevant feats we get the Prestige Classes: Diplomancer, Eldritch Warrior, Knight of the
ChoadChord, Song Mage.
Next up, Soul Magic. Imperative magic is Terry Pratchett's idea of a spell with its own sentience with a will to get itself cast. Declamatory takes over the role of a magic scroll, allowing the caster to cast something s/he normally couldn't, at the cost of INT drain and/or maybe WIS and CHA. So consent is involved, or not. The third one is Extemporaneous, more "like a wish". By it you, perhaps not even an arcanist, may cast any spell you want; but you take an ability-score hit when you do. This is how Monte punishes you for running a low-magic campaign, should he ever let you run one. Maybe Tevaeral in BCD.
Monte then gives the bard and bardlikes something to do in Spellsongs. Three tiers: spellnotes, spellchords, spellmelodies. Twenty pages of full-on spells follow; Monte lets Malhavoc narrate his design-notes in this chapter's sidebars.
Having filled up so much space on these new character-classes and spells, and since Soul Magic was arguably in the treasure genre, Monte only has five pages for physical magic items. Although here's the Book of Eldritch Might itself. That and the Black Grail will be very important for later work.
New monsters lower the curtain on this one. First are two types of Arcane Angel: one still good; the other more like a Thune Dervish, but for arcana. Then the Eye Golem, another construct. Finally the Unholy Riven: a mortal fully corrupted by arcana, like an in-house bodak.
Here Monte ties the eldritch with Moorcock-style planes in the multiverse. Malhavoc is chatty in the first three planes, but comments only once each on the last four.
The Nexus. This connects sites of arcane might across the Monteverse, starting with the six noted below. BCD will connect this selfsame Nexus with five more planes.
Bastion of the D'Stradi is actually a bastion against these demons. D'Stradi proper has already been overrun, and mutated, so here in Inshabiv they protect their own planet from the D'Stradi foothold here. Something like Maldev and other planes from Q1: Queen of the Demonweb Pits.
Pool of Glenmasis for fey.
Vabrin's Forge where the ettin Vabrin, his good head anyway, hammers out magic items Monte-style.
The Vale of Stars where starlight collects as dew and sinks into magical cisterns. Dragons are involved here also.
Tomb of Frozen Dreams is a glacier whither dreams fly away and get frozen into the ice.
City in the Storm is an oligarchic concern where people fly around on floaty whales. Someone been watchin' Fantasia!
The fonts are different too. Much easier on the eyes except for being slightly smaller.
Two notable spinoffs are The Book of Roguish Luck, by Wolfgang Baur 2005; and Beyond Countless Doorways by... well, by everybody. Roguish Luck is aimed to the Rogue class but it contains much eldritch as well, starting with the Gutter Mage and the Shadowsworn. BCD is effectively BoEM3.5 to the point of including The Nexus as a planar gateway, although with only one feat (and Malhavoc doesn't speak). Even the fonts are the same.
Late in 2001 Monte posted "The Ranger Revisited", as OGL. This didn't get bound into these books on account it wasn't Eldritch enough although, of course, the ranger gets "a slightly more generous spell progression and spell list". In 2003 the author overhauled the fonts to make it all look like an out-take from The Nexus and BCD.
Dragon #308, which featured draconic spells and items, let Monte expand his draconic spells and even to reference that BoEM 3 itself.