When Halo-Charged Skaab enters the battlefield, each player mills two cards. Then you may put an instant, sorcery, or battle card from your graveyard on top of your library. (To mill two cards, a player puts the top two cards of their library into their graveyard.)
The card you put on top of your library, if any, may be one you just milled, or it can be one that was already in your graveyard.