"Void", known as "Purgatory" (煉獄, Rengoku) in the Japanese version, is an archetype of Spell and Trap Cards. The first support for which was introduced in Crossed Souls.
There are two distinct themes among "Void" cards: Spells and Traps that support the "Infernoid" archetype, and cards that support the "Infernity" archetype (which were used by Kalin Kessler in the manga). There are also two members that do not specifically support either archetype: "Into the Void" and "Void Trap Hole"; however, "Into the Void" features "Infernity" cards in its artwork and fits into their playstyle.
"Unformed Void" is not a "Void" card because its Japanese name does not contain 「煉獄」. In addition, due to the opposite reason, "Blast Magician" is also a member of this archetype. However, because TCG/OCG support for this archetype only works on Spell and Trap Cards, this is not an issue, unless they are treated as Spell or Trap Cards (such as by the effect of "Relinquished" or "Psychic Megacyber").