The Spread of Void Sigils

The Man in Black and Denvel used a forbidden Igdias' ritual to amplify the Void Sigil, a technique that causes their spread.
Once spread, the Void Sigil becomes even stronger and is difficult to eliminate in the short term.
Even more dangerous, if multiple Void Sigils are placed in the same area, they will amplify each other, ultimately leading to the entire region being consumed by the Void Mist.