The history of Elves and Shadow-elves
This history is from the perspective of the Elves (for the purposes of Mystara, Elardrin are Elves). Since you have just captured a shadowelf, I figured it would be good to explain how the view the surface world, and how surface elves would view them.
This is an appbreviated history from the Gazatters. Things in brackets  are my editorial comments.
5000BC: 1st elf civilization on Southern Continent (now under icecap) [If you think of Mystara geography in terms of Earth Pangaea continent, then this would be Antarctica]
3500BC: Blackmoor flourishes. Elves trade and war with Blackmoor. [You recall that Rhedrian mentioned Blackmoor taking a part in the explosion of his ship]
3000BC: Planetshift. Blackmoor blows up. Elven home continent freezes over, becomes southern icecap. Survivors of the elven colony in Blackmoor flee int the newly-formed Broken Lands and underground. Temporary ice age in Glantri. [Broken lands border Darokin and Glantri, now very similar to Mordor, not sure how this colony got from the Southern Continent to here, but are clearly a different colony]
2800BC: New elven nation on the southern continent land of Vulcania [southern part of south America/Africa in Pangaea continent] divides on magic versus technology. Returnists under Ilsundal leave for northern continent on long march.
2200BC: Some [southern] Elven migrants settle in frozen valleys of Glantri. [look for the elves that lived there in 3000BC but do not find them]
2100BC: Ilsundal’s migration reaches Sylvan Realm [approx. Alaska]. Meditor and Verdier clans leave Ilsundal’s nortword migration and settle in southern Karameikos.
1950BC: Some underground elves come back to surface in Glantri, but do not meet the Sylvan elves.
1800BC: Ilsundal becomes an Immortal, creates the first Tree of Life.
1700BC: [Slyvan] Elves find artifacto of Blackmoor in the Broken Lands and trigger a cataclysm [causes clouds of ash and smoke to cover area for years] [Slyvan] Elves flee Glantri. [other elves return underground]
896BC: Group of adventurous shadow elves venture to surface. But few survive the strength of the hot sun of the Broken Lands and only a few return.
800BC: Alfheim colonized by followers of Mealiden, Elven wizards begin systematic alteration of climate to increase fertility of the land. Ice recedes to the far north.
792BC: Another group of Shadow Elves is sent to the surface. Arrive at a harsh location in the Broken Lands and conclude that the surface world will never recover from the cataclysms.
700BC: Mealiden is acclaimed King of Alfheim.
350BC: Maliden leaves throne to try to become an Immortal. Alevar of the Grunalf clan becomes king of Alfheim.
250BC: Maliden becomes an Immortal.
100AC: Celedryl of the Erendryl clan becomes king of Alfheim. [still current king]
131AC: After meeting a group of humans encounters the Shadow Elves underground, the Shadow elves learn that the surface is inhabitable. The king of the Shadow Elves sends a delegation to the surface. They meet King Celedryl. Initially, Celedryl was happy to welcome the shadow elves to Alfheim. He felt that some accommodation could be made, space could be found. Then he learned that the shadow elf population was equal to that of Alfheim itself!
Moreover, the shadow elves demanded reparations for their long neglect in the tunnels beneath the Broken Lands. The felt outraged that they had been abandoned to a harsh fate by their surface-dwelling cousins. They demanded more than half the land of Alfheim, even though they knew they were reconciled to their underground lives and th sunny summer of Alfheim hurt their skin and eyes. The final straw was their demanding the leadership of the country! Th shadows elves, abandoned for centuries, left no room for debate or compromise. Celedryl and the Alfheim clanmasters refused their demands. The shadow elves returned to their homes, threatening war.
150AC: Shadow elves incite underground humanoids to attack Alfheim, but they are driven back.
390AC: Shadow elves try again using humanoids to attack Alfheim.
550AC: The beast man [gnolls?] invasion of the wizard Illodius scars the magical forests. Alfheim Town founded in the blighted area.
560AC: Alfheim/Darokin alliance crushes the shadowelf invasion.
600AC: Slyvan Lands are conquered by Moorkroft. Feadiels arrive in Alfheim.
675AC: A shadow elf infiltrator, concluding that the surface elfs were good, friendly people, confesses to King Celedryl that many shadow elfs have infiltrated Alfheim, and provides names. King Celedryl purges shadowelf infiltrators from Alfheim, but is only partly successful.
700AC: Doriath, a former adventurer, assumes the throne of Alfheim. The Erewan faction of Erendyl clan leaves Alfheim for Glantri.
1005AC: current date.