The Crystalina bonus round rule is one of the many rules that can be applied during the Bonus Round episodes. It generally spawns around floor 4 or 5 as the Thief, but it can very rarely show up on floor 2 for all other characters.

Crystalina

Keep unused dice until your next turn. Lose 1 hp for each dice you keep.

When the rule activates, the player is allowed to keep any dice they haven't used on that turn for the next turn. However, the player will lose 1 health for each unused dice, which can be fatal if you have more dice than usable equipment.

The effect activates as soon as the turn ends, with all unused dice disappearing and then reappearing for the next turn after the previous turn's dice have been rolled.

The damage done by the health loss can be mitigated with the Reduce status effect: 3 counts of Reduce will allow you to store at most 3 dice without losing any health.