The ascendant plane is simply an alternate dimension, and throne worlds are just pieces of that dimension that have been sectioned off and claimed by powerful beings. The hive are the most adept at it, but others have made throne worlds such as the Mindbender and Mara Sov. Even a guardian could make a throne world given they were well-versed enough in hive magic and had killed a sufficiently powerful being (Such as Oryx at the end of King's Fall). Toland has even criticized players who defeated Oryx for not creating a throne world out of his death and using it to control the taken, thereby leaving a power vacuum within the hive and taken. Taken are born in the ascendant plane, which is why they resemble each other. Crota had no power over the taken, which is why his throne world looked different compared to most of the others we encounter.

<Riven's heart is not the same as the Heart of the Black Garden. If you want a ball that has the same textures as the Heart, look no further than the Centurion Axion Dart. Riven has no Throne World. Throne Worlds are carved from the Ascendant Plane. The Mindbender probably did not know the full extent to a Throne, and so only made himself bigger. The Ascendant Dreaming City is just a copy thrown into the Plane, and was left unchanged. Eleusinia, the Shattered Throne, was taken over by Dul Incaru, who probably didn't care for the well-being of it. Not to mention that the Taken spread Blight literally everywhere, which could contribute to its appearance. Also, they were ransacking the place for secrets. I assume that just slathering yourself in Blight should allow you to enter the world that lays beyond it. Not to mention that the rules of Ascendance changed because Mara decided to cheat and use Riven to create Eleusinia in the first place.>

The Black Heart was basically made of the power of the Darkness (stolen from the Hive by Quria during the Osmium Throne Incursion), which is the same stuff Taken are made of. Riven was Taken by Oryx, that’s why it’s got the same Taken vibes as the Black Heart. Throne Worlds and Ascendant Realms are the same thing, and they always have been. The Ascendant Realm/Sea of Screams is where powerful beings make their Throne Worlds. All Throne Worlds we had seen before were the Hive’s (Oryx’s and Crota’s), so they looked like Hive places. Riven was inside Mara’s Throne (which she created by wishing on Riven. Cheater…). Mara’s Throne was connected to the Dreaming City, and looked like the Dreaming City. Because Oryx sent the Taken to invade the Dreaming City and Riven was Taken, the Dreaming City and Mara’s Throne World are infected with Taken. That’s why it looks like that.