The great Empire of Shantiri was completely destroyed during the War of the Ancients. Very few survived, the alliance of the Ancient races was broken, the Nagas left forever, and the remaining races decided not to be friends anymore. They reunited only in the Alliance – a union gathered against a common enemy – the demons. And such an alliance is created again and again during each lunar eclipse. If I'm not mistaken, the skull of Sar-Elam and its other "parts" were also hidden by the Shantirians for a long time, but I might be wrong.
In general, this empire has a very, very interesting history. And I'm very sorry that its history, as well as the history of Ashan in general, now fits on just a couple of pages. ;(
Regarding Armageddon, I think it's more of a plot device here, but the meaning is roughly the same as in this map.