"The Armies of the Slain"
Pluto canoodled and conspired with Lorelei to create a new Valkyrie and then exploit her power over the dead. Amassing a cadaverous army to storm the daylight realm above his Underworld.
With the original Defenders held immobile at Pluto's mercy, it takes the intervention of the supporting non-team members, plus a second Valkyrie to free them.
Valyries lock horns and one of them is revealed as Lorelei, sister of the Enchantress. Duped by Pluto and abandoned in amnesiac form on Earth. The Asgardian sorceress has a score to settle with the Olympian Underlord.