In the terms of headcanon of the entity known as Master
To best describe this thing it is one of many things or weapons created to challenge Void Termina in the ancient past. Due to Void being an entity of pure chaos, Master was created by magical faction of the Ancients to personify emptiness or nothingness to combat it, afterall the chaos of life and existence can only be stopped when nothing is left. It initially began as a small relic or totem that one uses to initiate a ritual, simply holding it up high and let the essence of nothingness seep into the body. The original main goal was to imbue an individual the powers of nothingness, the ability to erase and to delete chaos or matter.
However due to the overwhelming nature of nothingness, it will quickly overtake the individual as a host, becoming the entity known as Master. With its’s programed purpose by their creators, it sees everyone and every living thing as chaos, (as chaos is seen as a mixture of soul, heart, dream, and dark, formulating an individual if the 4 matters are balanced). Thus Master was tasked to eradicate all of existence to cleanse chaos itself.
Thankfully through unknown means that managed defeat this horror, the being itself was sealed away after its host was killed. It is not known how it was defeated, but due to the creation of this being, it tarnished the good reputation of the magical faction of the Ancients. Thus it was deemed a matter most dark, a subject or event that is better left unknown and hidden away.
If there was a pause screen for this it would say:
“In the vast sea of space, every light shall be erased. The chaotic beating heart of every star would be snuffed out. The whole universe would be nothing but an endless void. But the only thing that lives stares at its new prey with red unblinking eyes...”
To describe the attacks, it basically uses the host as a vessel to conjure its abilities. Essentially utilizing the capabilities of its hosts to its advantage whilst infusing them with its power. The main body of the individual would be engulfed under a black void, appearing as a shadowy silhouette that can amorphously shape and contort its form into any capability. Thus with it possessing Magolor, Master has full reign of his abilities whilst empowered by the essence of nothingness.
Should the host’s body is damaged or in critical condition, Master would engulf its own host to preserve itself in order to persist within the world. To further protect its vessel, it would gravitationally pull various objects such as debris to encrust itself into a large planetoid or structure. In said form, it would launch debris laced with black corruption in order to defend itself from any attackers. Even to burst out shadowy tendrils to swat attackers.
In its final form, Master’s attacks are more towards shapeshifting. The circle would shape itself to various weapons or creatures in order to both overwhelm and destroy its enemies. In addition, the eyes of Master would not deviate on looking at the viewer, as its eyes would still stare at you regardless of its movement. But like many soul-based bosses, it does have their movesets, although the being itself would split itself apart as it becomes the attack itself.