Takara-Tomy have made one other update to their website today - they've added a page for their next Transformers Prime store exclusive, Darkness Unicron
, now referred to as Nightmare Unicron. According to the story text on the linked page, Nightmare Unicron appeared when the spark of Gaia Unicron was sealed away and he represents the still partially unsealed "true" incarnation of the Chaos Bringer, the story is a little fuzzy on this detail by our translation.
As speculated, Darkness / Nightmare Unicron is going to be a black version of the Chaos Bringer, so in his Gaia Armor form he'll blend well with Darkness Megatron or Nemesis Prime.
The page also specifies the launch of a contest, the "Second Arms Improvement Contest", a customizing contest for Japanese fans. It runs from 9th of January to the end of February. There are various prizes including a sound stage with Optimus Prime voice clips, a new Arms Micron Shining R.A. O and giveaways of Nemesis Prime. By our reading Nightmare Unicron isn't being given away through this contest, so it's likely this is the Darkness Unicron teased for a March release a while back as a Yodobashi Camera store exclusive, only available at the Akihabara (Tokyo) and Umeda (Osaka) branches.