You must face Eisenritter the Fallen, who has appeared from the flow of time and space. Hall of Fallen Knights is limited to three successful departure per week. You won't be able to use certain skills.
The battle features a stat gate, requiring achieving certain stats limits to enter the battle. The stat gate isn't the same for every region and varies according to publisher's decisions.
Only "pure" stats are counted, excluding any buffs or Vanguard stat increases.