The Lucubra are Diagon's minions. Many Lucubra appear as enemies of Sir George in the Ben 10: Ultimate Alien called "Solitary Alignment".

According to Gwen, a Lucubra is a demon from another dimension sealed by the first Forever Knight using a spell. 

Physical Appearance

A Lucubra has a green and gray gelatinous body with red teeth like markings in its body, resembling some kind of lizard. They use tentacles to feed on its victim's thoughts and uses them to communicate with others, once a person has been fed upon, the Lucubra can send them commands telepathically.

The Lucubra seem to range in sizes, one being as large as a dog, while another was several times larger then a human.

Powers and Abilities

A Lucubra has the ability to feed on the thoughts of humans and possibly other species. Gwen states that it feeds on the part of the mind used to pray or meditate until there is nothing left of the victim's mind. The victim then becomes it's obedient serant, evident by the turquoise eyes, which it is implied it can control from other dimensions.