What is the Legend Lore spell actually doing "under the hood"?

Question

I've been wondering how the spell legend lore actually works, and the best way I can come up with to phrase the question is to use the phrase "under the hood". I know how the spell works in game, as a player and a DM. I know what kind of info to expect when I cast it, and I know what kind of info to give as the DM. But what I am looking for is an in-universe explanation of what the spell is doing with respect to the world, the magical weave, etc. The spell descriptions for 5e and 3.5e both tell you only what kind of information the spell can provide, and give no insight into what is going on "under the hood".

Is there any reference from any edition that gives more detail about what this spell is doing? Sources can include any canonical sourcebook, any canonical setting, from any edition, and even canonical novels from canonical settings. My thought was that it taps into some conscient property of the weave, that the weave is somehow observant of the universe. But that is entirely a guess, and an unenlightening "it just works cuz magic" answer would not be surprising.

Answer 1

Originally, it let you learn about already existing legends

Legend Lore has its origins in Greyhawk, not the Forgotten Realms where the concept of The Weave stems from. It was published in 1976 in the first expansion to the original Dungeons and Dragons game, Supplement 1 - Greyhawk.

The original text for the spell is minimalistic and does not provide any kind of "under the hood" workings, other than that a great conjuration effort is required:

Legend Lore: By means of this spell the magic-user seeks to gain knowledge of some legendary item, place or person. (It is necessary for the referee to decide if the sought-after thing is in fact legendary in the scope of his campaign.) Such a spell requires great conjuration, so the time necessary can range from days to weeks (roll percentile dice, 01 = 1 day — 00 = 100 days). Often times the information gained through the spell will come in the form of a riddle or a poem.

The version in the 1e PHB goes deeper (p. 85)¹: the text here makes it clear that in spite of maybe spending weeks in casting the spell, it may not even give you the information outright, merely where you can find it. And it only provides information about legendary material that is available:

When completed, the divination will reveal if legendary material is available. It will often reveal where this material is - by place name, rhyme, or riddle.

This means if cast normally, it would only be able to retrieve information that has already been turned into some kind of legendary material. If, say, you are a lich and someone stole your phylactery, it would not yet know where it was hidden, even if you are legendary or your phylactery was, as no legends have yet been written about the theft yet, no songs composed.

There was also a matching bard ability called Legend Lore, which provided a percentage chance that a bard would "know something about a legendary person, place or thing" (p.118), that supports the idea that the spell delivers knowledge about existing, knowable legends, it does not create it:

Due to training, a bard has knowledge of many legendary and magical items (...). If some legendary knowledge is appropriate and the dice score indicates that the bard has knowledge in the area, then his or her ability will deliver information similar to the magic-user spell, legend lore (q.v)

So the spell in general tapped into existing myth and legend, and worked as a worldwide magical library or oral tradition search.

There only is an exception if the object is directly at hand. In these cases, the spell could generate new information, but only in cryptic form that you would have to interpret. No rationale for how that part works is given.

It will sometimes give certain information regarding the person, place or thing (when the object of the legend lore is at hand), but this data will always be in some cryptic form (rhyme, riddle, anagram, cipher, sign, etc.).

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¹ In a way, 1e AD&D can be seen as a compilation of the rules that evolved over time from the original D&D rules and additional supplements, as played in Gary Gygax' home Greyhawk campaign, with a few extra bits like psionics added. Therefore, it is useful to consider the rules text for spells from these two sources together to understand how the spells were conceived and worked originally, although they could undergo some evolution based on the campaigns' play experience.

Answer 2

The Weave of Magic (5ePHB 205 sidebar)

The worlds within the D&D multiverse are magical places. All existence is suffused with magical power, and potential energy lies untapped in every rock, stream, and living creature, and even in the air itself. Raw magic is the stuff of creation, the mute and mindless will of existence, permeating every bit of matter and present in every manifestation of energy throughout the multiverse.

Mortals can’t directly shape this raw magic. Instead, they make use of a fabric of magic, a kind of interface between the will of a spellcaster and the stuff of raw magic. The spellcasters of the Forgotten Realms call it the Weave [...]

When characters use divination spells such as detect magic or identify, they glimpse the Weave. [...]

Legend Lore requires you to name or describe a person, place, or object, all of which (per the 1st paragraph above) is permeated by raw magic, aka the Weave, in the Forgotten Realms.

Legend Lore is a divination spell, so it works by allowing the caster to glimpse the Weave itself, seeing that raw magic directly which is, or was, a part of that legendary person, place, or object.

(So yes, your supposition that the Weave is "observant" of the universe is close, except that it is the caster that "observes" the raw magic that is/was inside these legendary things, via the Weave.)

Answer 3

The AD&D 1e spell description gives different casting times depending on the availability of prior information about the person place or thing (PHB 84-85, 1980 printing):

Explanation/Description: The legend lore spell is used to determine information available regarding a known person, place or thing. If the person or thing is at hand, or if the magic-user is in the place in question, the likelihood of the spell producing results is far greater and the casting time is only 1 to 4 turns. If detailed information on the person, place or thing is known, casting lime is 1 to 10 days. If only rumors are known, casting lime is 2 to 12 weeks. During the casting, the magic-user cannot engage in other activities other than routine- eating, sleeping, etc. When completed, the divination will reveal if legendary material is available, It will often reveal where this material is — by place name, rhyme, or riddle. It will sometimes give certain information regarding the person, place or thing (when the object of the legend lore is at hand), but this data will always be in some cryptic farm (rhyme, riddle, anagram, cipher, sign, etc.). The spell is cast with incense and strips of ivory formed into a rectangle, but some item must be sacrificed in addition — a potion, magic scroll, magic item, creature, etc. Naturally, legend lore will reveal information only if the person, place or thing is noteworthy or legendary.

I view this version of the spell as a form of investigation mechanics for this edition of D&D. When the person place or item is present, the casting time is short, and it behaves more like a magically augmented investigation check -- the caster looks for hidden Elvish writing on an item, or carefully examines the location for details pointing to its legendary past. Longer casting times are consistent with the idea that the caster is doing magically augmented research -- pouring over ancient scrolls, contacting other planes (which is a lower level spell) or sages etc.

In AD&D 1e, mechanically, Legend Lore provides a way to implement investigation checks for legendary knowledge; stylistically, it provides a way of representing the "wizard carefully examines and object, discovering its secret past" and "wizard locks themselves away to find hidden knowledge" tropes.