Original: A stroke in 1857 left the king partially paralyzed and largely mentally incapacitated, and his brother William served as regent from 1858 until the king's death in 1861, at which point he acceded the throne himself as William I.
Simplified: After a stroke in 1857 the king was partially paralyzed and largely mentally weak, therefore his brother William became regent from 1858 until the king's death in 1861, then he became king himself as William I.