Just because this was easy to pull up, here's some relevant quotes about the sheer scale of Commorragh.
"In the depths of the webway lies Commorragh, named by many in fearful whispers as the Dark City. Commorragh is no mere metropolis, for it is to the greatest cities of realspace as a soaring mountain is to a mound of termites. Its dimensions would be considered impossible if they could be read by conventional means, its population greater than that of whole star systems."
"Commorragh appears within the webway as a composite entity of impossible scale, a shimmering, contradictory realm that plucks at the sanity of those who approach it. Thousands of ships dock each day within its outflung spines, for the Dark Eldar are far more numerous than even their craftworld kin suspect."
"Commorragh today is an endless nest of architectural contradictions and spatial anomalies. Each of its estates has been overdeveloped to such an extent that their growth has been forced into thevertical plane, the rival regions sprouting upwards like a tangle of needle-plants fighting for a scrap of sunlight. Each of the spires and towers is linked to its fellows by hundreds of curved arches and strands, and crested with complex silver structures that glow with stolen energies. Its towering aeries and palaces reach both upward and downward, spiralling into the depths of captive space. With every passing year, the parasitic city seeks to devour ever more of the hidden dimension that acts as its host."
"Far above the glinting metallic peaks of Commorragh are the Ilmaea, or 'black suns', dying stars ablaze with poisoned light that were harnessed at the height of the Eldar empire."
This is what Yvraine did by just ascending.
"Yvraine's possession by the macabre god Ynnead had shaken the very fabric of the Dark City. Far away, a miscarried ritual conducted by Eldrad Ulthran on the crystal moon of Coheria had twisted Yvraine's fate ― chosen by forces unknown, she had died at the exact moment of the god's ascension. This confluence of Empyrean energy and realspace flesh was so severe it led to a hyperspatial quake known in Commorragh as a dysjunction. Dozens of spires toppled and districts turned in on themselves, skyscraping statues and high towers shivered and fell apart. Millions died, but there were worse fates in store for those who still braved the streets."