Since Elder Dragons are such a hot topic again, I as inspired to write an article I had been thinking about for a while now: an appraisal of all sources dealing with Elder Dragons, to see what we actually know about them. The sources are often old or obscure, and thus a lot of people have only heard about it indirectly, which leads to Chinese whispers-style mistakes. For example, look what is being presented as fact over on Magic's TVTropes page: "Nicol Bolas ascended as he fought the other four Elder Dragons, allowing him to win their war and become the last surviving one." Yeeeeeeeaaaaaah... No.
You can find the article I wrote here. I'd cross-post the entire thing onto here, but that would require me to edit in all the links to sources and upload all the scans from the old Acclaim comics, and I'm simply to lazy to do that. I'll just do a quick TL;DR version below. If you want the rest of the article, with the full discussion and all the scans and quotes I've based my analysis on, you can click the link

The TL:DR version of the article:
Some stuff that might surprise you about the sources: The old comics tell us nothing about what they actually are and don't mention the Elder Dragon War. We do learn that the Elders are old, powerful and that they can travel between planes. The War, the fact that Elder Dragons are a race, that the losers became Elder Land Wurms etc. comes from an old tearaway calendar that is, as far as we know, lost to us. (It is not the one Gavin Verhey has been posting pictures of, that one doesn't match the description we have of it) This information was saved by Jeff Lee, who put it on his website. Lee's website is also where we learn that the Elder Dragons dominated the entire multiverse, that their War spanned multiple planes and some other minor facts. Unfortunately it also contains at least one mistake (regarding Sivitri Scarzam being active during the Dragon War) and one big assumption (Piru and Scarzam being Sub-Elders. Lee has later stated that this came not from any official source, but was the consensus opinion of the early storyline forums). Luckily later appearances of Nicol Bolas in the Legends II cycle, the Time Spiral cycle, and even his bio on Magicthegathering.com back up a lot of stuff from Lee's site, though there are also some surprising omissions.
What we actually knowThat Elder Dragons are ancient, the progenitors of all draconic races and are immensely powerful is mentioned many times in Legends II and Time Spiral block. The only official source that states they can travel to other planes is a text piece in the Elder Dragons comic (which is pre-revisionist, but not contradicted elsewhere. Unless you count Doug Beyers recent statements about the Elders being from Dominaria). The fact that there was a War and that the five from Legends were the only survivors is actually only mentioned in Nicol Bolas' bio on the Mothership! Yes, the only official and still extant source about the War stems from around the time of Alara!
The fact that the losers of the war became Elder Land Wurms, and the ancestors of all other wurms, is never mentioned in official sources, except in the now-lost calendar, so the only extant source we have on that is Jeff Lee's site, which is technically not official.
Stuff we have no proof ofWe don't know what on earth is up with Piru. The Acclaim comics call her "The Sixth Elder" time and again, but Bolas' bio on the mothership states that there were only five survivors. (Though it is likely that was written by someone who never even heard of Piru, considering how long ago her appearance was). She could be a Sub-Elder, but we don't have proof for that.
We also have no idea about Nicol Bolas's ascension. It seems very likely he ascended during the climax of the Elder Dragon War, but that's not stated anywhere. I've often heard it said that whatever ended the war caused his ascension, or that he ascended and used his new powers to end the War, but there is no proof of either claim.
There is no proof of the Elder Dragons having an empire. Jeff Lee mentions them "dominating Dominia", but doesn't mention whether that takes the form of an empire, a loose coalition or independent Elders ruling over independent realms. Heck, it could even just mean they were the strongest beings in the Multiverse rather than be a reference to them ruling any kind of states. Lee does use the words "civil war" to describe the Elder Dragon War, which you could argue sounds like there should have been one Elder Draconic state, but remember that this site is not an official source. We have no idea whether WotC at the time thought of the Elder Dragon War as a civil war, or if it was just a phrasing Jeff Lee liked.
There is no proof of a Worldspell turning all the losing Elder Dragons into Elder Land Wurms. Jeff Lee's site only mentions them being "stripped of their power", but doesn't say if this was done on a Multiversal level, a planar level, or even a case-by-case basis. The only reference to a Worldspell I've been able to find was on the MTGSalvation wiki, on the timeline page. The reference to it was put in by an anonymous editor who did not cite any sources.
There is no proof of the Elder Dragon War lasting millennia. In fact, we know nothing of how long it lasted at all. An article on Dragons on Phyrexia.com claims the war lasted "epochs", but that article is riddled with mistakes, and other articles by the same author show a lot of fanon being passed of as canon, so we should ignore the article in question.
...unless there actually is proof of any of the above in a source I'm missing of course. I've done my best to find every single source that mentions the Elders and their War. I think I found them all, but you know how it is with Magic, there could always be some obscure online story you've missed, so if you find I've missed a source, do let me know, then I can add it in!