Wallow thyself in ashes - Violent distress is accustomed to find relief in eccentric actions, and thus the wallowing in ashes shows that Jerusalem’s grief is unbearable.
The spoiler - Nebuchadnezzar.
Related Commentaries of Jeremiah 6:26
Jeremiah 6:26
26 O daughter of my people, gird thee with sackcloth, and wallow thyself in ashes: make thee mourning, as for an only son, most bitter lamentation: for the spoiler shall suddenly come upon us.