The Black Garden is a separate dimension not of any known space or time. It houses the Black Heart, an entity that is intrinsically linked to the Darkness, and is tended and guarded by the Vex. Long before any living being ever set foot in the Garden, tales of its existence were told throughout the system. In particular, the Warlock Pujari had a vision of the Garden when he drowned in the waters of the Shores of Time. Pujari's vision mentions that the Black Garden's existence is related to the Traveler's coming to Mars. However, he's careful to point out that "This is not the beginning but it is the reason." The Traveler did not create the Garden, but it's actions may have allowed the Garden to come into existence, because "In these things there is always symmetry." Pujari adds, "The Garden grows in both directions. It grows into tomorrow and yesterday. The red flowers bloom forever. There are gardeners now. They came into the garden in vessels of bronze and they move through the groves in rivers of thought." The gardeners in question likely refer to the Vex, though the same word is used in a somewhat different context in Ghost Fragment: Mysteries 2, which suggests that the Gardener is a powerful being that may predate the Vex. After learning of the Black Garden's existence from the Exo Stranger, the Guardian travels to the Reef to inquire the Garden's location. Queen Mara Sov and Prince Uldren reveal that a Vex portal on Mars leads to the Garden, but requires the Eye of a Vex Gate Lord to access. After obtaining the Eye, the Guardian activates the Vex spire controlling the portal and then enters the portal itself. Deep in the Garden, the Guardian discovers the Black Heart, a malevolent entity guarded by the Sol Progeny that is feeding off of the Traveler's Light, keeping it dormant. Once the Heart is destroyed, Light returns to the Traveler and it begins to heal. The Garden's dimension ceases to drift and becomes part of Mars, making it accessible to the rest of the Guardians. Later, a powerful Vex Axis Mind called The Undying Mind arrives in the Garden and attempts to revive the Black Heart with assistance from the Sol Primeval. The Mind also attempts to separate the Garden from our timeline, but it is destroyed before it can do either. During the Taken War, Oryx, the Taken King sent an expeditionary force of Taken into the garden to investigate and possibly restore the Black Heart. An Echo of Oryx and its Taken coven invaded the Garden through secret tunnels under Freehold. The Sol Divisive was overwhelmed by the Taken as the Garden became a war zone. Only through Guardian intervention were the Taken expelled.

A pocket reality, outside of space and time where the Vex were created

I don't even have time to explain why I don't have time to explain.

The black garden was the Vex's response to the hive worship of the darkness. During the battle between Quria and the Vex and Oryx and the hive, the Vex learned sword logic, but even that was not enough to defeat the hive. So they looked to what else they did that they didn't. It was worshipping the darkness. What The Black Garden is or who created it we still don't know.

I'm sorry. I can't give you information. [spoiler]because there are too many details for your taste. [/spoiler]

Big heart. Very scary. Vex enemies live. Heart give life. Heart die vex die. Vex go boom. [spoiler]the end[/spoiler]

I think the black garden was made by the traveller and its 'gardener' was the traveller. Vex being vex took it over or perhaps were the Gardeners made by the traveller, which became self aware. The black heart is the vex attempt at recreating the darkness to worship it for power as they discovered when they invaded crotas realm. Just my thoughts

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