Sun-touch

An ancient spell put in place by a powerful sorceress to act as retribution should the Fire Elves suddenly die out.
This event is triggered at the end of the first book and permanently alters the surface of the planet. Only a few within the impact zone survive. Many more will suffer displacement, insanity, or death from it's aftereffects.
See "Lakes of Flame" by Blue Stahli

Effect

A spike of plasma from the nearby sun strikes the surface of the planet where the Fire Elves reside. Temperatures are high enough that nothing survives without magical protection, the ground melts into pools of lava and takes days to cool.

Side/Secondary Effects

The sudden injection of heat into the atmosphere causes massive alterations in weather patterns in the short term. In the long term, the adjustment of weather patterns, boiling of nearby oceans, etc. cause ecological shifts affecting every group of elves. In addition to the deaths suffered at the impact, this is ecologic shift becomes the true retribution, as elves are forced from their homes to prevent insanity.

Manifestation

Upon the death of the last Fire Elf in the region, the spell activates, triggered by an altar deep underground. At that time, a magically pointed solar flare erupts from the nearby star. In a few minutes, elves on the ground notice a huge streak of light, with those right under the impact zone noticing it far less than those with a much better angle (such as elves in the Southern hemisphere or living atop mountains). Within minutes of noticing, the plasma impacts the planet's surface, instantly killing 99% of elves within the impact zone. The only ones alive would be elves with some form of magical protection that would actually be effective against something so devastating. Currently, there are only two elves who's gifts would prevent their deaths.

Source

Many cycles ago, when the Fire Elves interacted more with mainland elves, there was quite a bit of discrimination and hostility toward the Fire Elves. One of their most powerful sorceresses created an enchantment, tied to a hidden altar. She provided proof of that the spell was in place to the mainland elves (although such proof has faded from history with the previous cycles). As a result, the mainland elves did not commit genocide against the Fire Elves in that cycle.

Discovery

It is only discovered by the elves of the current cycle when they commit genocide against the Fire Elves.