Conversation so far:
What was a famous work of Paul Klee's that came out during the Mystical-abstract period?
Paul Klee created abstract works in that period such as Abstract and Farbige Kreise durch Farbbänder verbunden.
What other painters were famous during the Mystical-abstract period, besides Paul Klee?
Paul Klee produced watercolor paintings in the fashion of Paul Cézanne and Robert Delaunays' cubistic form concepts.
Was World War I important for Paul Klee in the Mystical-abstract period?
Paul Klee continued to paint during the entire war and managed to exhibit in several shows.
Who were Paul Klee's colleagues during the Mystical-abstract period?
Paul Klee produced with August Macke and Louis Moilliet watercolor paintings.
What else can you tell me about Paul Klee's Mystical-abstract period?
Paul Klee produced watercolor paintings, which implement the strong light and color stimulus of the North African countryside in the fashion of Paul Cézanne and Robert Delaunays.
What famous work did Paul Cezanne produce during the early 1900s?
Pyramid of Skulls, c. 1901. The dramatic resignation to death informs several still life paintings Paul Cézanne made in his final period between 1898 and 1905.
What famous works did Robert Delaunays produce during the early 1900s?
In the prime of Robert Delaunays' career he painted a number of series that included: the Saint-Sévrin series (1909–10) and The First Disk (1913).

Question: Why was the Mystical-abstract period special?
Self-contained rewrite: Why was the art of the Mystical-abstract period special?