Dark Mirror Hole
The Dark Mirror Hole was the secret weapon of Count Graduon and Mary Lee Walsh in their invasion of CWCville, seen in Sonichu #5 and #6. Although they intended to trap Chris in its otherworldly reflection, his twin sister Crystal Weston Chandler wound up trapped in his place. Since this dark day, the primary mission of Sonichu and the Chaotic Combo has been to free Crystal from the Mirror. You know, when they're not lounging around on the beach or screwing each other's brains out.
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Both the Dark Mirror's name and style are swiped from Sailor Moon. The name is a complete ripoff of the Black Dream Hole that appears in the Sailor Moon Super S movie whereas the mirror itself and its behavior are associated with one of the villains of the television series, who used mirrors to capture the protagonists in the first Six Episodes of Sailor Moon Sailor Stars and was herself trapped in a mirror in Sailor Moon SuperS. There is doubt as to whether or not Chris has actually watched any of these, or if it's second/third-hand information from Megan.
Christian's rendition looks like a purple photo frame with what could be construed as Mickey Mouse styled ears.
By concealing the Dark Mirror in a street on the battleground, the villains lured Chris-Chan Sonichu to that exact spot, whereupon Graduon activated the Dark Mirror Hole spell to cause Chris to sink into the mirror. Not the sharpest knife in the drawer, Chris walked right into the trap.
As Chris-Chan Sonichu sank into the mirror, his transformation was canceled, reverting him to his human form of Christian Weston Chandler. However, before he was completely engulfed in the mirror, Sailor Megtune and Crystalina Rosechu managed to grab hold of his hands. Apparently, Graduon hadn't counted on anyone actually caring enough about Chris to try to save him, which is an understandable error.
Realizing that Chris would suffer more if his sister and gal-pal were trapped in the mirror instead, Graduon made the magic pane of the mirror flip over. While Megtune was too focused on Chris to notice, Crystalina sensed the mirror flipping at the last second, and used her tail to fling Megtune out of danger, so that only she would become trapped in the Dark Mirror.
Like Chris, Crystalina reverted to her human form inside the Dark Mirror. Seizing on the opportunity, Mary Lee Walsh demanded that the heroes of CWCville surrender unconditionally in exchange for Crystal's safe release. Before Chris could make a decision, Patti-Chan and Darkbind Sonichu arrived to break the standoff. Using her magic shamrock vibrator, Patti rescued the Dark Mirror from Walsh's grasp, and Darkbind and Sailor Megtune forced her and Graduon to withdraw.
Following the battle, Chris and his hedgehog allies worked diligently to free Crystal from the Dark Mirror, but the only thing that had any effect was a Sonichu Ball. Magi-Chan quickly concluded that only the combined power of all seven Sonichu Balls could extricate Crystal from the Dark Mirror. Those, or the Chaos Emeralds. Whichever one's easier.
Since then, the main plot of the comics has been the hunt for the Sonichu Balls. As of Sonichu 8, five have been collected, with two more in Chris's possession as he floats aimlessly through the time-stream. Chris was slated to return to CWCville in Sonichu 9, after which Crystal's freedom would be a simple matter.
In fact, it was far simpler a matter than that - in Sonichu 10, Chris, under the form of Collosal Chan simply walks into the mirror, picks up Crystal, and walks out. This is probably an act of shoddy damage control on Chris's behalf - he most likely realised that he had forgotten to pick up on the plot device, and needed to rectify the plot hole as quickly as possible.
Inside the Dark Mirror
Because of Chris's "less is more approach" to art, the dimension within the Dark Mirror Hole is only depicted as a blank purple void (Similar to the Time Void used in following issues). It is left to the reader to assume that the dimension is filled with vile, Lovecraftian demons who gang-rape Crystal day and night and force her to subsist on a diet of her own excrement.