Boiga gocool
Range
Bhutan, Bangladesh and India (Assam, Sikkim, Arunachal Pradesh and elsewhere in the north-east)
The arrowback tree snake is a rear-fanged colubrid found in Bhutan, Bangladesh and India (Assam, Sikkim, Arunachal Pradesh) (Wikipedia, general species account).
The Reptile Database also lists Manipur, Meghalaya, Nagaland, West Bengal, Odisha, Uttar Pradesh, Mizoram and Tripura. The account used here gives no further detail.