Horsemanship (This creature can't be blocked except by creatures with horsemanship.)

Sacrifice Xiahou Dun: Return target black card from your graveyard to your hand. Activate only during your turn, before attackers are declared.

Rulings

If a turn has multiple combat phases, the ability can only be activated before the beginning of the declare attackers step of the first combat phase in that turn.2013-09-20
Despite the similarities between horsemanship and flying, horsemanship doesn't interact with flying or reach.2009-10-01
You can't activate Xiahou Dun's ability to return Xiahou Dun itself to your hand. You must choose a target before you sacrifice Xiahou Dun, so it's not a black card in your graveyard yet.2009-10-01