Night Hunters. They were called Night Raiders in RTW.
They're as ahistorical on a battlefield as arcani and flaming pigs.
Tacitus did say the Harii painted themselves black and worked at night. But they did not fight on battlefields during the day covered in black. This is not a unit for the battlefield, but is covered by the assassin agent.
Arcani is a ridiculous spelling of the Areani: who operated as Roman agents among British tribes. Also, this is a unit covered by the assassin agent.
And flaming pigs only exist in accounts of RUMORS from the siege of Megara. And none of the authors who report it give the rumor any credibility.