Niflheim was the Shadowrealm belonging to the Elder Hel. The realm was the prison of Nidhogg before it was by Dr. John Dee when he killed the Yggdrasil with Excalibur. In Norse mythology Niflheim was the realm of the dead unworthy to join in the feasting of Valhalla, reached by a rainbow bridge called the Bifrost.
When Josh Newman stabbed Nidhogg with Clarent, he glimpsed a memory of when Yggdrasill was dying while the creature was in still imprisoned in Niflheim. The description depicts a landscape of black rock and pools of red lava, with filthy clouds from which the roots of the Yggdrasill hung. Nidhogg then notices that the roots of the Yggdrasill are whitening and pieces breaking off as the roots die.