From the Sledgehammer Games Blog:
“The battles have been fierce, and the victors stand strong. As 2015 begins, it’s time to up the ante with the Call of Duty Clan Wars Diamond Division. The first Diamond Division engagement will take place from Friday, January 30th, 12PM PST through Sunday, February 1, 12PM PST.
Opt In and Compete with the Best
Clans with at least two 1st place wins in the Platinum Division will have the option to opt into Diamond Division via the Call of Duty Companion app following our next app update. After your Commander opts-in, your Clan will compete exclusively against other Diamond Division Clans at the highest level of Clan Wars competition.
If your Clan has not yet earned two 1st place Platinum wins, you can still opt into Diamond Division at a later date if and when you do fulfill the requirement.
New In-Game Gear: Valkyrie
Along with Diamond Division comes a whole new set of in-game gear. Clans that place 1st in Diamond Division will unlock the first piece of the prestigious Valkyrie set, with subsequent 1st place Diamond Division wins unlocking more pieces.”
Source: Beachhead Games and Sledgehammer Games Blogs