Wretchlings are adaptions of Hollow Men, but instead of foul gases, they are filled with solid entrails, and instead of their skin being made out of paper, it is made of skin, leather and fur stitched together, making them many times more deadly. They are summoned by Warlocks.
They are first encountered when The Dead Men travelled through France.
Later, Charivari's army of warlocks used vast numbers of Wretchlings in The Battle of Roarhaven, using the creatures to mass attack the walls by claiming on ropes created by the warlocks' light-magic.
Stephanie, Valkyrie's reflection, details most of the conflict with the Wretchlings during the battle of Roarhaven, and they appear to be strong and capable of intelligent fighting in combat. Stephanie kills dozens of them with the sceptre of the ancients.
In Midnight, Skulduggery Pleasant shoots several Wretchlings with a sniper rifle, using special bullets designed to dismantle them. When Valkyrie questions if they can feel any pain Skulduggery states that even though they have brains, they're stolen from the dead, twisted and warped and attached to the Wretchlings like parts of a machine, and the only thing on a Wretchlings mind is the messiest way to kill someone.
Wretchlings carry swords or daggers, as well as being very strong and fast. They do seem to have some sort of fighting ability, as some of them manage to meet the combat abilities of Valkyrie Cain's reflection.