Because of the never-ending series of battles of the time, Madara eventually began to suffer from overuse of the Mangekyō Sharingan, leaving him blind. In order to regain his sight he took Izuna's eyes for himself, granting him an "eternal" Mangekyō Sharingan with which he led the Uchiha to prosperity. Madara insists Izuna consented to the act.
Itachi Uchiha suggested that his eyes had been taken by force; however, Madara insisted that Izuna had willingly given up his eyes for the good of his brother and the whole clan, seeing that Madara needed to remain strong to protect the clan from its growing number of enemies. Izuna later died honourably in battle, fighting to the death even in his blind state.
from narutopedia