Death rains from above - Airburst Rockets. After about 60 yards the main rocket explodes, damaging nearby enemies and releasing six more smaller rockets downwards. In PS3 version rockets released will go in all directions and may even come straight back towards you or go upwards. These smaller rockets will detonate after a period of time.
Nidhogg is the name of the mighty Dragon of Death from Norse mythology, who constantly gnaws at the roots of the world-tree Yggdrasil. It is told that Nidhogg exists even beyond Ragnarök (twilight of the gods) to collect the corpses of the fallen and take them to the underworld.