This is something which has been on my mind for a while.

In every faction there is some moment which truly defines an army, be it a statement, battle or even a scene in a book. The Space Marines have many examples from "And they shall know no fear." to the war for Rynn's World, the Orks have two Wars for Armageddon and their ongoing battles with the Tyranids, and Chaos has its own examples from Dark Apostle's conflicts to the Abyssal Crusade. What I want to ask though is, given their comparative lack of attention, what do you think the Eldar have which defines them?

To give a personal example: In my opinion it actually comes from an Imperial Guard book, namely Ghostmaker from the Gaunt's Ghosts series and the conflict on Monthax. It's better known for being a series of short tales but the main conflict actually has something rather interesting in that it involves a small band of Eldar manipulating the Ghosts into obeying them. This consisted of a single Farseer and a very small number of Dire Avengers if my memory serves me correctly. They were present on the world to help prevent Chaos potentially gaining access to a Webway Gate, sealing it off before leaving the world. As the Eldar could not take the casualties needed or had the firepower required to repel a full army, they psychically altered several platoons' perceptions to make them think they were fighting on Tanith as it was burning.

Now, this eventually failed and it did leave to some dubious lore when it comes to Farseers and psykers, but the eventual culmination of the conflict is having the Dire Avengers, Ghosts and another regiment taking on a much larger Chaos army. The book follows this up with this (sorry, this is an abridged version):

"Years later.... Imperial tacticians.... would be utterly unable to account for the success of the action. .... There was no sense to the data. Simple statistics should have had Gaunt's expeditionary force cut down to the last man.... They slew, approximately, two-point-four thousand soldiers of the enemy. .... The tacticians would decide that the only explanation could be that there were no enemy units on the field that day. .... Only then did the computations and the statistics and the possibilities match up."

This might sound strange, but overall I think this perfectly represents the Eldar. Why? Because they accomplished their goals, defeated the primary threats and left without the greater Imperium ever knowing of them. Better yet, a small number of Aspect Warriors, which the aforementioned tacticians had no knowledge of, were enough to turn potentially mass slaughter of Guard into a complete rout for Chaos. It's one of the few times where, in their own right, Aspect Warriors are truly treated as the Eldar equivalent of Space Marines as they deserve - Few in number, but in the right place with the right number of supporting troops they are capable of truly astounding things.