Host of the Phoenix King
The Host of the Phoenix King is one of three Elven special armies released as part of The End Times. Specifically, the Host of the Phoenix King refers to the "loyalist" faction, comprised of High Elves and Dark Elves who choose to swear loyalty 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, supplemented by Wood Elves.
Canonically, this army defeats the Aestyrion, a splinter faction led by Tyrion and based on those High and Dark Elves refusing to accept Malekith's new rule, and the two hosts are amalgamated to form the Host of the Eternity King.
For the tactica, see: Warhammer/Tactics/8th Edition/Host of the Phoenix King