It opens at level 40. You can gain 1 practice seal, for a shot at opening one of the 5 treasures.
Every 10 levels you unlock another fight to earn an extra practice seal to open another treasure. This continues up to level 80, at which point you can open every treasure in the breakthrough for that day. Gaining some rare ninja fragments and usually awakening materials.
You MUST spend your practice seals that day, you can't save them for future days. They will disappear on reset.