You control at least six other creatures that can attack this turn.
An opponent is unable to block and kill your creatures.
Steps
Declare
Hellkite Charger and six other creatures you control as attackers, triggering
Druids' Repository seven times and
Hellkite Charger.
Resolve all
Druids' Repository triggers, putting seven charge counters on it.
Activate
Druids' Repository seven times by removing seven charge counters from it, adding ({5} magic symbol) ({R} magic symbol) ({R} magic symbol) .
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.