You need to fight Leandra either way (the option to do the soul forge would have come up at the very end). Check the loot around her body, or check the inventory of whoever picked up that stuff.

It isn't the Death Knight Bane spell you need for the Portal to Eden. Leandra drops (or gives you, if you perform the sould forge) a 'Death Knight Control Wand', which you need a character to equip, so you can use the spell 'Death Knight Removal'.

Leandra is better off dead, considering what she did... also
if you perform the soul forge and bring the sisters with you to Eden, Leandra takes control and switches sides in the final fight. You can choose not to take them with you, but either way you are down one party member. You are better off with Leandra dead, so Icara will be in the final fight, along with Zandalor.