Dune-Brood Nephilim does not have summoning sickness.
An opponent is unable to block Hellkite Charger and Dune-Brood Nephilim.
You control at least eight lands.
Steps
Declare Dune-Brood Nephilim and Hellkite Charger as attackers.
Hellkite Charger triggers, causing you to pay ({5} magic symbol) ({R} magic symbol) ({R} magic symbol) to untap all attacking creatures you control and to get an additional combat phase after this one.
Deal combat damage with Dune-Brood Nephilim.
Dune-Brood Nephilim triggers, creating a 1/1 Sand creature token for each land you control.
Move to your next combat phase.
Declare Dune-Brood Nephilim and Hellkite Charger as attackers.
Resolve the Hellkite Charger trigger, causing you to pay ({5} magic symbol) ({R} magic symbol) ({R} magic symbol) to untap all attacking creatures you control and to get an additional combat phase after this one.