How did The Undertaker get in a coma?

its a storyline used in 2010 the undertaker suffered an orbital bone injury the after a 619 from Rey Mysterio the storyline was that the undertaker was brutally attacked by his kayfabe half brother kane which put him in a coma or vegatative state kane would then go on to win the world heavyweight champion the undertaker would later return at summerslam to confront kane culminating in their feud reigniteing