Creature cards in graveyards can attack and block as though they were on the battlefield, can block or be blocked only by creature cards in graveyards, are Zombies in addition to their other types, and have undeathtouch. (If they would deal damage to a creature card, exile that creature card instead.)
1/19/2018 During combat, creature cards in graveyards can attack and block just as if they were on the battlefield. Attacking creature cards in the graveyard can be blocked only by creature cards in the defending player’s graveyard.
1/19/2018 Creature cards in graveyards can attack planeswalkers on the battlefield.
1/19/2018 Other combat-relevant abilities such as flying work on attacking or blocking creature cards in graveyards. An attacking creature card with flying in a graveyard could be blocked only by creature cards with flying or reach in graveyards.
1/19/2018 Over My Dead Bodies allows you to attack or block with creature cards in your graveyard. If you do, for the duration of combat, the attacking and blocking creature cards are treated as if they are creature cards on the battlefield which means they can be affected by spells or abilities that would affect creatures on the battlefield. That means, for example, if you have a Lord of Accursed on the battlefield (“Other Zombies you control get +1/+1”), all your attacking and blocking graveyard creatures (which Over My Dead Bodies makes into Zombies) would get +1/+1 while in combat.
1/19/2018 As you choose attackers or blockers, check whether state-based actions would immediately remove any of them from the battlefield. If so, those creatures can’t attack or block, as appropriate.
1/19/2018 Creature cards in graveyards attacking or blocking will cause abilities that trigger whenever a creature attacks or blocks to trigger.
1/19/2018 You can target attacking or blocking creature cards in graveyards with spells like Giant Growth.
1/19/2018 If an attacking or blocking creature card is destroyed by a source without undeathtouch, such as Murder, it stays in the graveyard but is removed from combat.
1/19/2018 If Over My Dead Bodies leaves the battlefield in the middle of combat, graveyard creatures go back to being just dead. They’re removed from combat.