Kirk, Knight of Thorns, is an NPC that will invade the Chosen Undead three times through their journey. His barbed equipment are his trademark and the source of his title as the Knight of Thorns.
First, Kirk will invade in the Depths. Later, in the Demon Ruins, and finally, in Lost Izalith. If he's defeated in these three occasions, his corpse will appear near the Fair Lady.
The Set of Thorns he wears describes him as a "notorious member of the Darkwraiths".
His Barbed Straight Sword describes him as "the infamous Darkwraith Kirk, also known as the Knight of Thorns for the gnarly spikes on his favorite weapon".
Design Works Interview
Game no Shokutaku Interview
Kirk's connection to the Darkwraiths and Chaos Servants