Guardians of Time Trilogy universe

In the fictional Guardians of Time Trilogy, author Marianne Curley constructs a highly detailed alternate universe in which much of the action of the series takes place, unknown to the regular world. In this universe, two organizations battle for control over time.

The Guardians of Time (known also as The Guard) is a society dedicated to preserving history against the attempts of the Order of Chaos to alter it. It is headed by a sexless immortal called Lorian, who is backed by a Tribunal of nine members, each a representative of a house.

The headquarters of the Tribunal, as well as the Guard itself, is located in Athens, year 200 BC, outside of the mortal measurements of time. For their purposes, they also use a place called the Citadel, connected to another area known as the labyrinth (also used by Order of Chaos) which serves as a disembarkation point for the Guards' missions into the past. Guard meetings frequently take place in Arkarian's, a Guard member's, abode hidden within the depths of a mountain. Connected to this mountain is the hidden city of Veridian and later learned, also connected to Neriah's fortress.

The Order of Chaos (known also as The Order) is the collective group of villains in the series. It was created by, and is currently led by Lathenia, the Goddess of Chaos, the Order seeks to alter the Prophecy predicting their fall by changing the events of the past. As the Goddess of chaos, Lathenia attempts to manipulate the past, in order to alter the present to create chaos (floods, famines, etc.) for her own benefit. Her ultimate goal is to gain control of the world, because she believes that she should have been born first, rather than her brother, Lorian. The identities of the members of the Order are kept secret, even from each other, so we don't find out who many of them are in the books. The Guard battles against the Order, trying to protect the present from their attempts at destroying it.

Realms are separate locations existing in alternate dimensions. To travel to another realm, a rift must be opened to it, allowing travel to the other realm. Only Immortals, or several other powerful beings acting together can create a rift. A single person with the given power of agelessness, which endows the receiver with some reduced immortal powers, generally cannot force a rift to open by themselves.

...
Wikipedia