Argentum is one of Reyna's automaton guard dogs, along with Aurum. Argentum is the silver hound, as his name means 'silver' in Latin, and he has eyes made of rubies. Both of the dogs have the ability to know when a person is lying and will then attack the person.
While not having a large role in the book, Argentum and Aurum follow Reyna both times she talks to Percy alone. They seem to dislike Percy Jackson greatly, growling at him almost the entire time they are in the same room as him. Reyna warns him that they will attack if she wishes it, or if he steals anything. She also warns that they can tell when people don't tell the truth and that they hate liars.
Argentum appears while protecting Reyna, Nico and Coach Hedge. While fighting ghosts in Pompeii, his head gets twisted 45 degrees and he loses an eye. However, the Amazons repaired him.
- Argentum is the only automaton in both series to be made of silver.
- Argentum and its counterpart, Aurum, may have been modeled after King Alcinous's gold and silver guard dogs, mentioned in Book Eight of The Odyssey.
- Argentum is not considered a weapon, as he is allowed to enter New Rome, but as Annabeth noticed, Argentum and Aurum could easily tear her apart with their sharp metal teeth.