When Guru Dronacharya was made the sentapati of Kauravas, he affirmed Duryodhana that he would be able to capture Yudhishthira and his whole army, but only on one condition that Arjuna should not be present there during that time.
After hearing this Duryodhana approached the Susharma (सुशर्मा), the king of Trigarta (त्रिगर्त) and asked his help to fight from Arjuna on that day.
Who were Samsaptakas (संशप्तकः) ?
Samsaptaka (संशप्तकः) means the one who has taken an oath (सम्यक् शप्तमङ्गीकारो यस्य)
As requested by Duryodhana, Susharma and many other king on that day took an oath to fight Arjuna, because of this oath all these warriors were together called as Samsaptakas.
Samsaptakas included Susharma(सुशर्मा) and his five brother named as Satyarath, Satyaverma , Satyavrata, Satyeshu, Satyakarma and many other kings who also took similar oath to fight Arjuna.
Who was Arjuna fighting, when Chakravyuha was formed?
On the thirteenth day, Dronacharya formed a Chakravyuha(चक्रव्यूह) so that Yudhishthira will get trapped and captured. Arjuna and Krishna knew how to break it so Samsaptakas distracted them and took them to the other side of the Chakravyuha
Arjuna was fighting the Samsaptakas on this day, when Abhimanyu was drawn into the Chakravyuha.