Daniel Drumm was the man in charge of the New York Sanctum when Stephen Strange arrived there after escaping the attack in Tibet.
Brother Voodoo is the name given to Jericho Drumm, who eventually becomes Doctor Voodoo later in the comics after actually taking over from Doctor Strange as the Sorcerer Supreme for a time.
When Kaecilius attacks the New York Sanctum he takes down Daniel first.
Derrickson seems to be implying that Daniel’s death could cause Jericho Drumm to react in some way, potentially making him part of a future sequel.