To get into the void, you have to find and activate 4 glyphs first. After that you can get into the void via each of these glyphs. The order doesn't matter which one you activate when, as long as you have clicked each one once.
The Four Glyphs and their location
Position from the Glyph in Somweyr
Position from the Glyph in the Sewer
Position from the Glyph in Ren'Takal near the Ruins