Hello everyone Achroma here bringing you an introduction to our newest permanent feature, mysterious cave!
Il do this in sections so please bare with me!
First of all the interface!

Now as you can see already by the interface there is several options of rewards aswell as several exploration options!
The main thing to note about these is that Keys are always used first, coupons secondly and ingots thirdly. Meaning if you have 50 keys the explore 50 times only removes your keys! This of course works in a similar way if you only have 25 keys since it will only cost you half the exploration cost in that case!
As for rewards the possible ones are Runestones all the way up to lvl 5! (I will go over them in a little bit) Aswell as purification runes and purification protection runes!
Now regarding the runestones, They are all li
nked to a specific item.
Initiative runestones can only be put into your headpiece like this.

Critical runestones are only for the sword.

Injury runestones only go into your chest piece.

Control runestones only go into your Scroll piece.

And of course the last runestone combo only goes into your necklace piece.

As you can see by the interface runestones work very similar to magatamas, except that they are not for your core stats but rather the sub stats that refinement also gives you.
Now the next question would be how do you ideally put your runestones?
This is very optional according to me. As you could put them all into your move 1 (usually main combo starter) depending on your team. But if your move 1 combo starter does not benefit from having additional combo stat since they cannot start one eitherway with their basic, then you essentially lose half of the effectiveness.
As such initiative runestones can be used to increase the slight initiative of your interupt or maybe even your secondary combo starter if you have one. And Combo can be used on whoever you feel needs to start a combo more frequent.
Critical is obviously for whoever you want to crit most and usually injury is paired together with critical.
Now how do i go putting in my runestones once i have them? well very similar to magatamas i personally believe that you should atleast fill one slot on each character first before improving them. Ideally you should get them up to 3-4 before focusing entirely on that one char you want to have the highest of them. Very similar to magatamas you will reach a point where its costly to improve them and thus you should move on to the next characters stones untill they reach that point and so on.
I will of course try to answer as many questions as im capable of if you have any just feel free to leave a message!
As for keys you can get some from the current events to get those runestones cheaper!
I hope this helps you and good luck! /Achroma