Irisviel is literally the human personification of the grail. The grail itself is a destructive force that will grant the owners wish, but at immense cost. Angra Mainyu, or Avenger, is another Servant hidden in the grail that causes chaos and destruction if released.

Kiristugu gets the choice of keeping the grail and letting it cause that ruin (but getting his wife and child back), or destroying it (but losing them).