Terminus

Terminus is the Roman god of boundaries, the protector of the limits both of private property, and of the public territory of Rome. He has no Greek counterpart.

History
He was represented by a stone or post, set up in the ground with the following religious ceremonies. A trench was dug, in which a fire was lighted; a victim was sacrificed, and its blood poured into the trench; the body, upon which incense and fruits, honey and wine were thrown, was then cast into the fire. When it was entirely consumed, the boundary stone, which had been previously anointed and crowned with garlands, was placed upon the hot ashes and fixed in the ground. Any one who removed a boundary stone was accursed and might be slain with impunity; a fine was afterwards substituted for the death penalty.

On the 23rd of February (the end of the old Roman year) the festival called Terminalia, according to Wissowa, a festival not of the god but of the boundary stones (termini), was held. The owners of adjacent lands assembled at the common boundary stone, and crowned their own side of the stone with garlands; an altar was set up and offerings of cakes, corn, honey and wine were made later, a lamb or a sucking pig was sacrificed. The proceedings closed with songs to the god and a general merrymaking, in which all the members of the family and the servants took part.

The worship of Terminus was either introduced to Rome during the reign of Romulus or during the time of his successor Numa.

The Son of Neptune
Terminus first appears when Percy headed to the Senate House and served as the city's guard saying no weapons are allowed and lets his assistant, Julia, take Riptide. Terminus inspects Percy with his "hands" which Percy criticizes. Terminus gets angered by this, but eventually allows Percy to enter the city. He later asks Hazel to move a rock in it's "correct" position.

After the Senate meeting, Percy, Hazel, and Frank headed to Caldecott Tunnel in which they meet Terminus again near the entrance. Terminus, again, "inspects" the three and allowed them to pass.

Terminus appears once more during the battle against Polybotes' armies. Polybotes accidentally enters the city's borders with weapons in hand thereby angering Terminus. Polybotes turned to Terminus and pushes him over destroying his body with the head still intact. Percy knocks the Giant over and grabs Terminus' head and heads to Polybotes. Percy threw the gods' head at Polybotes thereby killing the Giant, since the Giants can only be killed by gods and demigods working together.

Terminus is last seen in the city border saying to Percy that he is allowed to pass through the city freely without any inspections from the god. Julia later appears giving them party hats on the god's orders.

The Mark of Athena
Terminus appeared on the Argo II as it approached Camp Jupiter. He appeared in an explosion that almost knocked Annabeth off the ship. He began to order them to not cross the Pomerian Line with weapons on board. Piper tried to charmspeak the statue, but he is unaffected by her powers and knocks Katoptris from her hand with a small explosion. He also insults Athena, saying that the Romans had a proper place for her, something that greatly upsets Annabeth. Leo at first thinks that statue may be an Automaton that someone wound too tightly and started to poke at the pedestal with a screwdriver. Terminus in response tells them to leave and surrender, which confuses Leo as he isn't sure if he means they should leave or surrender.

Terminus says that they should do both, but Jason said they were on a peace mission. Terminus still stands his ground, saying that the entire ship is a weapon, and so it can't cross the Pomerian Line. Annabeth comes up with a plan, telling Leo to keep the ship hovered above the camp and that they would climb down with a road ladder. Terminus agreed to the terms and allowed them to enter as long as the Argo II stays in the air.

Trivia

 * Terminus has no Greek counterpart, but Charon is the Greek god of boundaries, as Terminus is the Roman god of Boundaries
 * According to Hazel he has OCD (obsessive compulsive disorder).