Wizards and witches often swear by Merlin, Morgana, or Circe. That is because they, and many others, are still active. Ascended, rid of the shackles of mortal life, they are now part of the Arcane, powerful, enigmatic beings with their own interests. Some benevolent, others less so. Many Arcane band together to form Houses, grouped by similar alignments and interests, while others are wild, independent.
The most ubiquitous form of divination, other than crystal balls, is card reading, using the Arcane Deck of Fate. Mysterious magical artefacts whose cards change depending on the situation among the Houses.
The Arcane pursue their own interests, but cannot be everywhere at once. They have servants and agents, they offer deals. Every person who made such a deal bears a mark of the Arcane somewhere on their body.
Voldemort seeks the Path of the Ascendant, his mark based on the principle of the Arcane marks, he seeks to upset the balance and found a new House, one that would rule supreme.