Albert Barnes Commentary Jeremiah 6:26

"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 destroyer shall suddenly come upon us." — Jeremiah 6:26 (ASV)

Wallow yourself in ashes — Violent distress is accustomed to finding relief in eccentric actions, and thus the wallowing in ashes shows that Jerusalem’s grief is unbearable.

The spoiler — Nebuchadnezzar.