what are they used for? it says recruiting people, does mean tavern or a few limited ones? I see people mentioning drark rukia. is that the only one that can be redeemed? also I have a bunch of them but cant find them in backpack is that normal? also some seem to be of the opinion that dark rukia is impossible to get without spending money on vitality. is that true?
It's specifically for the currently active event, like before with Dark Rukia. It's the Ultimate BP event or whatever. The thing for this event is Sogyo no Kotowari, the zanpakuto. It buffs all ally attack damage while Dark Rukia buffs damage rate, so the two are mostly only good (But perfect) with each other.
Click on the "Ultimate BP" button in-game there you can see your crystals and you can claim rewards.
Dark Rukia and the current one, sogyo no kotowari (zanpakuto), are impossible to get without buying gold.