Okay, 'tis been almost a week since Ch. 507 did its catastrophe to the fandom, but... well, HERE GOES MY SPECULATION
And yes, it's about Yama-ji's bankai :3
So, his bankai is Zanka no Tachi , which literally means Long Sword of the Remaining Flame.
I've seen in the forums(was it BA or Deathberry I can't remember D8) that this bankai is... weird because neither the name NOR the kanji of the shikai(Ryūjin Jakka) and the bankai is similar. We usually have similar shikai-bankai names, like Zangetsu as Tensa Zangetsu, and Senbonzakura and Senbonzakura Kageyoshi. In Gin's case, his shikai Shinsou sounds nothing like his bankai Kamishini no Yari, but there is a smiliar kanji in Shinsou(神鎗) and Kamishini no Yari(神殺鎗).
But in case of Ryuujin Jakka(流刃若火) and Zanka no Tachi(残火の太刀), neither applies.
HOWEVER(I'm getting off-topic here), if I thought about his bankai's ability in relation to his shikai, no matter how unrelated shikai-bankai thing...
Coming from his shikai call, "Reduce All Creation to Ash;"
His sword, compact form of all the almighty flames that used to cover the entire space, will reduce anything, ANYTHING it touches, into ashes; people, buildings, ZANPAKUTOS, etc. There is nothing that can counter it, THE strongest flame-type zanpakuto in the world.
Now, imagine how terrifying that would be. Your zanpakuto, your hand, CRUMBLES RIGHT IN FRONT OF YOUR NOSE D8