What exactly the Black Heart is is unknown, but the most plausible and well supported theory I've seen is it is a work larvae captured from the Hive. The Vex are searching for what they call an "emperor to all outcomes." Put more simply, they want to collect as much data as possible so they can simulate every possible scenario, so they can develop a response to every possible outcome an ensures it ends with Vex victory, or at least survival. Because of this the Vex must work on trillions upon trillions of projects to satisfy victory conditions for the trillions upon trillions of possible scenarios. The Vault of Glass and Black Garden where two such projects built to fulfill two different victory conditions. The Black Garden is built as a result of the "worship" function that Quria developed during its war with Oryx. It learned that by directing "worship" at the worms it captured granted great power. Before being defeated by Oryx it transmitted what it learned to the Vex collective, and they used that information when creating the Black Garden.

Well the most interesting thing about the Vex is that they learned worship from the Hive. They built the Vault of Glass as their attempt at a throne world. Long ago, when Oryx and his sisters were still together, Savathun convinced Crota to cut open a portal to a place which was supposed to have great power. So, Crota being the over achieving son that he is went to Orxy's throne to show off to Ir Halak and Ir Anuk. He cut open the portal and Vex came flooding through. At first they studied his throne world, then started building in it. They spent a few years studying the Hive. They learned worship from observing Acolytes pay tribute to Oryx. That is why when you walk into the Black Garden you see them "praying" to the Heart.

One thing I haven't quite wrapped into a complete thought yet, is we were researching the vex during golden age. Before the collapse, they were on Venus building the citadel. They weren't shooting at us or attacking us during the golden age. They were builders. Time is tricky with the Vex, but the post collapse vex are aggressive and have adapted to worship, the very things they learned from the hive. So it seems like at least in our timeline, the vex adapted the hive sword logic sometime after our collapse. So what was the vex purpose and nature pre collapse? What were they doing here?

Yes and no, while they are both servants of the darkness, and both carry the beliefs of the darkness. They are more akin to rivals ir even enemies by the darkness own definition. [spoiler]to elaborate, the darkness and the hive follow the principles of the sword logic, which means to love or befriend someone is to try and kill them and if they die you become stronger, but if you die you make them stronger. And to the darkness this is both their greatest gift and curse. They show theur love through death in a way where the saying, "what doesnt kill you only mames you stronger" , takes on a litteral meaning in that by you surviving and becoming stronger makes you worthy in the eyes of the hive and of the darkness.[/spoiler] We see them interact and show how the vex first came into contact with the hive and with the darkness. And of how after spending enough time in oryx's throne world the vex begin to adopt and adapt to the sword logic of the hive. [spoiler]it is known that both species strive to attain a perfect form, a form so perfect that it could survive even the heat death of the universe. I mention this because the methods they both use to attain this goal are the same, service to the darkness.[/spoiler] As for the ahamkara their appear to be 2 or even 3 seperate known groups. The worm gods that the hive worship that serve the darkness and work to spread its influence. The ahamkara that made deals with gaurdians and with the city which as far as i can tell are really just making deals as a way to accumulate power for themselves. And the third group is pure speculation on my part, in that some ahamkara could potentialy be allied with the light. I believe their may be some 'good' ahamkara out their because of a passage from the books of sorrow. It tells of how durring a hive war against another race, that race prayed to the dragons for their wishes to be heard. While it isnt known if the dragons are the ones known as the worm gods or if they are a different sect. Oryx does show great displeasure in this saying that he should put the dragons(worm gods) all in cages.