There are two choices, either the leader of the Dark Elves Malekith, or his mother (I think) Morathi.
Their ultimate goal with regards to the vortex is to capture Scrolls of Kerarti to start the rituals required. Their units are a mix of ‘standard’ elf spear and bow (albeit crossbow) units, as well as some more exotic units like the witches and hydras. I know a fair bit about the Dark Elves from the lore essentially that they are very angry Elves that feel slighted by the High Elves due to an age-old enmity. Be prepared for me to make some awful decisions throughout though! The very rough plan I have is to 10 turns blow-by-blow per article. As a test though I thought I’d try a game as one of the races that may not suit my playstyle, the Dark Elves. So far I’ve tried playing as the High Elves and Lizardmen both of which I’ve just about managed to win the campaign with. Since its release, I’ve played a lot of Total War: Warhammer II (TWW2 for shortness) and it is one of the best games I’ve ever played.