From 1d4chan

The Aestyrion, also known as the Host of the Aestyrion, is one of three Elven special armies released as part of The End Times. Specifically, the Aestyrion refers to a splinter faction, comprised of disgruntled High Elves and Khaine-worshipping Dark Elves, who break away when Malekith is declared the true Phoenix King of the High Elves and an enraged Tyrion draws the Widowmaker to become the Avatar of Khaine.

Canonically, this army fights against Daemons, Beastmen and the Host of the Phoenix King before Malekith triumphs, defeats his rival and the two hosts are amalgamated to form the Host of the Eternity King.

For the tactica: Warhammer/Tactics/8th_Edition/Aestyrion