Their leader was Malekith the Accursed.
Ages ago, they were destroyed by the Frost Giants including Malekith in a war between Jotunheim and Svartalfheim. Now their realm was in ruins and filled with their remains and tortured souls of the Dark Elves.
On Midgard, Malekith had returned as a half-alive and half-dead Dark Elf on his quest to find the Casket of Ancient Winters and then use it to wipe out humanity and resurrect his people from the dead and then make Midgard their new home. But his plans were foiled by Iron Man, Thor, and Black Panther.
On Svartalfheim, Black Panther had been pulled into their realm after destroying the stone. As he examined their remains, he was chased by a swarm of Dark Elven souls. After a long chase, he finally found his way out through the well-like portal which led to Alfheim where he bumped into Faradei and Hawkeye.