An opponent is unable to block and kill Savage Ventmaw and Hellkite Charger.
Steps
Declare Hellkite Charger and Savage Ventmaw as attackers.Druids' Repository's first ability triggers twice, and Savage Ventmaw and Hellkite Charger trigger once.
Resolve both Druid's Repository triggers, putting two charge counters on it.
Activate Druid's Repository by removing a charge counter from it, adding ({1} 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 and to get an additional combat phase after this one.