Complex sentence: He was "dethroned" temporarily in 1858 by Paul Morphy, who announced his own retirement from chess in 1859, and permanently in 1866 by Wilhelm Steinitz, who dominated chess until his own defeat by Emanuel Lasker in 1894.
Split into simpler sentences: He was "dethroned" temporarily in 1858 by Paul Morphy, who announced his own retirement from chess in 1859. In 1866 he lost to Wilhelm Steinitz, who dominated chess until his own defeat by Emanuel Lasker in 1894.