The hill range that separates the state of Manipur from the state of Nagaland is know as:

The Barail Range (or Barail Hills) is the significant hill range that separates the state of Manipur from Nagaland [t1][t6]. While the Purvanchal region consists of various ranges like the Patkai-Bum, Naga Hills, and Manipur Hills, the Barail Range acts as a crucial geographical divider between these states and also separates Manipur from Assam to the west [c1][t4]. The Barail Range is recognized as the highest hill range in Assam and extends into both Nagaland and Manipur [t1][t3]. In contrast, the Patkai Hills and Naga Hills primarily form the watershed between India and Myanmar [t2]. The Arakan Hills (Arakan Yoma) are located further east, primarily on the border of Nagaland and Myanmar, extending southward into the ocean [c1]. Therefore, the Barail Hills are the specific range defining the boundary between Manipur and Nagaland [t9].

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 2: Physiography > 5. The Eastern Himalaya > p. 17