Ghost

The spectres were ancient spirits that inhabited the Ark of the Covenant. It was unknown if they were the souls of previous victims of the Ark's wrath or if they were divine beings themselves.

As Belloq and the Nazis opened the Ark, the spirits emerged from the chest and flew around and through the awestruck soldiers. When they flew near Indy and Marion, they simply bypassed them because their eyes were closed.

At first appearing to be benevolent or in Belloq's words," Beautiful!" the spirits suddenly transformed into sinister angels of death, effectively sealing the fate of Belloq and the Nazis. The spirits went back into the Ark and combined to form a fiery ball of lightning that shot all the Nazi soldiers dead and incinerated Belloq, Dietrich, and Toht.