From Halopedia, the Halo wiki
At some point in Forerunner history, Triniel was settled as part of the ecumene and became a flourishing world with multiple cities on all three continents and a Builder facility for creating and repairing Forerunner ships. However, during the Forerunner-Flood war, Triniel became a target for the Flood. With no escaping their fate, no help forthcoming and nowhere to run, the planet's population chose to commit mass suicide by releasing a planet-wide toxin that would deprive the Flood of their food source and potential for enormous growth. By the time the Flood arrived, Triniel was a dead world and was thus left untouched by the Flood. Over the next 100,000 years, the toxin dissipated, leaving Triniel's atmosphere safe again. The effects of the toxin likely created a divergent evolutionary path for all of the planet's flora and fauna.
During his time as the Monitor of Installation 04, 343 Guilty Spark learned of Triniel and its fate. In 2557, during his mission to find the Librarian, Spark attempted to take the Ace of Spades to the Etran Harborage to get an upgrade seed, only to learn that the shield world had been destroyed and to then witness the destruction of its remnants. As a result, Spark decided to take the Ace of Spades to Triniel instead to use its Builder facility to get the upgrade seed. Spark offered Captain Rion Forge his help in exchange for her help at Triniel and Earth as he would need a Reclaimer to unlock the facility. Rion ultimately agreed to the deal.
In July 2557, Ace of Spades arrived at Triniel where the crew marveled over being the first humans to see the planet and all of the salvage they could make off of the world until Spark revealed the fate of its inhabitants. Upon arrival at the facility, Rion's touch as a Reclaimer powered it up again and Spark used the activated control console to begin rebooting Triniel's communications satellites in an effort to be able to access the Domain. Spark used a translocation pad to transport the crew deeper into the facility where he retrieved a slipspace flake, but warned that he could not allow the rest to be salvaged due to the dangers they presented. Spark took the crew to the lab where the design seeds were created and designed an upgrade seed for the Ace of Spades while the crew salvaged from the level despite feeling that they were salvaging from a tomb or mass grave.
In one of the rooms in the facility, the Ace of Spades crew found the remains of the five Forerunners who had stayed behind to shut it down as the planet committed mass suicide and listened to the translated version of the final message they recorded. Having completed their task, the crew left the planet. Knowing that the activation of the satellites would eventually draw others to Triniel, Spark shut them down again before they could be rebooted, allowing Triniel to remain a lost planet, in a lost star system, in a lost sector of the galaxy and thus free from mass exploitation as it deserved as a mass grave. With the slipspace flake and upgrade seed obtained on Triniel, Spark was able to greatly upgrade the capabilities of the Ace of Spades.
After their successful mission to find the Librarian, the crew of the Ace of Spades was left wondering what to do. With the salvage from Triniel, the Ace of Spades would never lack for credits or resources again. However, Rion wasn't sure that they would ever return to Triniel as plundering a planet-wide grave site didn't feel right to do the first time, let alone a second time. Rion declared to the crew that with their Triniel loot to provide funding and their upgraded ship, they had vast possibilities of what they could do after they found the UNSC Spirit of Fire.
Topography, ecology and climate
A planet one and a half times the size of Venezia, the gravity is a little higher than optimal for humans, but was manageable for humans to operate on its surface. Triniel has azure oceans and three continents of emerald. In the early days of the planet, the long, narrow continents had been a connected series of volcanic mountain ranges that wrapped like a spiral around the planet. In time, the range broke into three pieces, eventually cooled and the volcanic activity slowed, allowing the evolution of a temperate, lush world ripe for colonization. By 2557, Triniel retained a flourishing population marine, mammalian and avian lifeforms including a species of bird that had evolved from the size of small birds to be as big as the Ace of Spades. 343 Guilty Spark theorized that the planet-wide toxin used by the Forerunners to commit mass suicide created a divergent evolutionary path for Triniel's flora and fauna.
Due to Forerunner colonization, Triniel possessed many Forerunner cities and a manufacturing facility for Forerunner ships built into a mountain at the very tip of the central continent. As a result of being abandoned for 100,000 years, the cities were somewhat taken over by nature, though they remained intact.
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