Mass Effect 2
A temperate world that has hit the "sweet spot" for carbon-based life, Horizon has a nitrogen-oxygen atmosphere maintained by abundant indigenous photosynthetic plants and bacteria. While the native plants are not very palatable to humans, the soil conditions are such that a handful of introduced Earth species have flourished, and the colonists must take strict care to prevent ecological disasters. Genetically-engineered "terminator seeds" that grow nutritious but sterile crops to minimize outbreaks are the rule rather than the exception.
Animals on Horizon appear to be exploding in diversity, similar to Earth's Cambrian period. Large flying insect analogs take advantage of the thicker-than-Earth atmosphere and low gravity to grow enormous. Microbial life has proven relatively benign; a series of vaccinations for the most virulent strains of soil-borne diseases is all that is required for a visit.
Mass Effect 3
A temperate world that has hit the "sweet spot" for carbon-based life, Horizon has a nitrogen-oxygen atmosphere maintained by abundant plants and bacteria. Its soil is particularly compatible with Earth vegetation, and so a thriving human community has sprung up.
Horizon's population is in flux. A Collector attack in 2185 inflicted significant casualties, but refugees have been streaming to the planet since the Reaper invasion began. Since many land illegally, there is no way to accurately track population. Although many colonists resent the newcomers, others have embraced Horizon's new role. Advertisements for "Sanctuary," its main refugee center, can be seen galaxy-wide.
A typical Terminus colony possessing minimal tourist value, Horizon promises substantial economic opportunity, especially in providing new products for humans and supplying the Turian Hierarchy. Surveyed 18 years ago, Horizon received pilot habitation four years later; the colony proper is now eight years old.
Blessed with verdant forests and abundant fresh water, Horizon maintains a colonial culture that thrives as a refuge from the increasing restrictions of Citadel-governed society. Horizon has attracted numerous dissidents, marginal people, and fringe-dwellers from across Alliance space.
A variety of Horizonian insects taste similar to Earth shrimp and can be used in dishes involving the latter. However, eating them apparently induces a mild laxative effect.
According to information gathered by the Collectors in 2183, the human population of Horizon is 643,315. Despite an Alliance defense outreach fronted by the efforts of either Ashley Williams or Kaidan Alenko, the Collectors make off with approximately a third of Horizon's colonists in 2185.
In 2186, a facility on Horizon called Sanctuary lures people in with the promise of shelter from the Reaper war. With all signal activity in the surrounding region of space being actively interfered with, the real reason Sanctuary has stayed off the Reapers' radar until late in the war is because it is actually a Cerberus operation experimenting in converting refugees into husks for their studies in indoctrination.
The Reapers invade only when Cerberus has made headway into co-opting the Reapers' control over their creatures, although by then the Sanctuary sham has been exposed to the galaxy at large due to Commander Shepard's interference. Most of Horizon's population that remained after the Collector attack went to their fates at Sanctuary.
Mass Effect 2
- Mission: Horizon (mission)
Mass Effect 3
- Mission: Priority: Horizon
- Despite being described in the Codex as a Terminus systems colony, Mass Effect 3's galaxy map shows the Shadow Sea Cluster as being in the Attican Traverse.
- In Mass Effect 3, Commander Shepard's line while departing from the planet ends with "I've had enough of this place.", similar to Shepard's last comment from Mass Effect 2 which is "I've had enough of this colony."