In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, the creatures and planeswalkers you control from Command the Dreadhorde’s effect are exiled.

Command the Dreadhorde deals damage to you before the creature and planeswalker cards are put onto the battlefield, but if this brings your life total to 0 or less, those creatures and planeswalkers enter the battlefield before you lose the game. Any abilities they have that trigger when they enter the battlefield won’t resolve if you lose the game, but their static abilities (such as that of Platinum Angel) may help you survive with 0 or less life.

If a card in a graveyard has {X} in its mana cost, X is considered to be 0.