When Darkblood is banished, Cecil uses three items: a reconstructed page from an ancient magic book, a candle, and a hand bell. Bell, Book, and Candle are three items used in a Roman Catholic ceremony of Excommunication, by which officials of the Church formally cut off a heinous offender from any Communion with the Church, ostensibly sentencing the person to eternity in Hell.
Bi-Plane, who was thrown into outer space by The Immortal in episode 1, is briefly seen as a frozen corpse as the Mars shuttle leaves orbit.
In one scene Robot grips Rex on the shoulder and pinches him.
This is a homage to Robot's voice actor Zachary Quinto, who played Mr Spock in the Star Trek movies. Mr Spock is famous for his Vulcan grip, a move in which Mr Spock stuns his enemies by pinching their shoulder.
The Sequids are a various homage to Starro, the mind controlling starfish from DC