Hamatulas, (hah-mah-CHOO-luh) the barbed devils, are a caste of Baatezu tasked with patrolling the first four layers of Baator: Avernus, Dis, Minauros, and Phlegethos. They all report to the Pit Fiend Gazra, who is the consort of Fierna, Phlegethos' ruler.
Hamatulas are the security guards of the Baatezu. They can be bribed or tricked, though I wouldn't count on it. They are not a direct part of the Blood War.
Sources:
Planescape Monstrous Compendium, Volume 1
Faces of Evil: The Fiends
Monster Manual (D&D 3.5)
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