Where am I supposed to get Gehenna for Lucemon's Satan Mode?

[Steel Otaku]

It's not in the store and all I'm left with is the Source of Darkness for his larve form. Are they ever going to add it back to the store? I tried looking up what the deal was, but I can't find anything.

[Dux]

Sometimes it gets released in the store during specific events/promotions. It's also possible to obtain them during in-game specific events, like Lucemon bosses, or even as event competition rewards. I won one on the 1st of November as part of the event reward.