On 23rd day of battle, when battle didn't conclude, Bhishma attempted to use the Prashwapastra against Parashurama. This weapon was not known to Parasurama and would put the afflicted to sleep in the battlefield. Before Bhishma could release it, however, a voice from the sky warned him that if he uses this weapon it would be a great insult towards his Guru. Pitrs then appeared and obstructed the chariot of Parashurama, forbidding him from fighting any longer. At the behest of the divine sage Narada and the gods, Parashurama ended the conflict and the battle was declared stale mate.(effect of any astra on parashurama is not clear as he is avatara of Lord Vishnu and chiranjivi).