Amergin against the druids of the Tuatha de Danann during the Milesian invasion is the first example of bard vs druid that comes to mind. Taliesin rescuing Elphin from Maelgwn was a good example of using magic. Manawyddan in the court of Caswallon as portrayed in The Song of Rhiannon is another good example of a powerful magician in a bard (that one is Welsh based).
Taliesin was the bard in Arthur's court until replaced later legends with Merlin, who was based on Myrrdin, and fulfilled the seer / prophet role.
The original bard was based on Celtic mythology and history (hence the jack-of-all-trades aspect and magical nature) with the fili as stated, but also with influence from the skald and jongleur. When Gygax changed that to the 1e appendix option it was because of fighting and skill aspects of a bard, and also because in mythology the Celtic bards learned magic from the druids (sidenote: other sources have claimed bards first taught magic to druids and then druids retaught it to bards who had forgotten). When Jeff Goelz remade the bard in Dragon Magazine 56 it was specifically influenced on Manawyddan as stated in the article.
The fili/skald/jongleur was clearly stated in the Strategic Review article introducing the class, with similar statements in future editions.
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Where I think you misinterpreted what I said is when I claimed the main influence was the fili that I was excluding other inspirations. The class evolved over time and 5e is meant to allow various archetypes under one similar class. At least that was one of the statements during the open beta testing. We aren't playing "Dying Earth the RPG" but that doesn't mean a player cannot take inspiration from them to build the character. I know many players who follow the jongleur concept, for example. This is from 2e:
"In precise historical terms, the title “bard” applies only to certain groups of Celtic poets who sang the history of their tribes in long, recitative poems. These bards, found mainly in Ireland, Wales, and Scotland, filled many important roles in their society. They were storehouses of tribal history, reporters of news, messengers, and even ambassadors to other tribes. However, in the AD&D game, the bard is a more generalized character. Historical and legendary examples of the type include Alan-a-Dale, Will Scarlet, Amergin, and even Homer. Indeed, every culture has its storyteller or poet, whether he is called bard, skald, fili, jongleur, or something else."
It still stresses the typical versions. I can make a biwa hoshi bard if I wanted to now. That doesn't change the roots of the inspiration for the class. This is from XGtE:
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And this is from the 5e PHB:
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That scholarly fili is still there showing it's roots. Same as the skald. What's missing is the jongleur, specifically called out as "not every minstrel or jongleur..." is a bard. The only identity crisis is with players denying what bards actually were in favor of other pop culture tropes. Claiming the medieval minstrel comes to mind is the opposite of what is stated. At no point has the bard ever been stated to be based on a gigolo that I know of, lol. That's more of the pop culture style. Playing A Bard's Tale springs to mind for that one. I
Considering I was defending bards as full casters (because caster level was the previous indicator instead of max spell level, among other things) then I think you acknowledging such in my model is actually a "proof is in the pudding" moment. ;-)
I'll also point to Vanya Hargreeves for the "music is magic" trope. Destroying the moon and creating an extinction level event seems appropriate.
The bottom line is the current incarnation matches up to the original inspirations better than past versions.
The strategic review articles quoted above, 2e PHB quote, and Dragon article should give some. A person could probably dig more up. Praise poetry, chronicling events, and oral tradition is typical of the role.
I didn't say Celtic bard in regard to Gandalf. I said bard. One source is listed above. Another inspiration is Väinämöinen from the Kalevala.
Snodgrass, Ellen (2009).
Encyclopedia of the Literature of Empire.
Infobase Publishing. pp. 161–162.
ISBN 9781438119069.
A lot of Tolkien's characters took inspiration from bard or bard equivalents. Väinämöinen had similarities to Odin and Merlin, and he would also use his songs and music for magic.
As for literature on the bards' roles, I'll refer you to:
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It's written by D.W. Nash and stamped by the University of Oxford November 7, 1939 -- it has the citations you are requesting to validate my claims. The real question I would ask is what counter evidence have you offered? ;-)