It is the ultimate weapon he received from his father Drona. He was told he should never use this weapon no matter how great the calamity he faced, but in response to Duryodhana’s death, he activated it against his better judgement. Although it was possible to resist this weapon’s world-stunning destructiveness with the full might of Arjuna’s arrows, it left scars so great that the legends say the whole area around them became barren and unproductive ground for 12 whole years.
Although this is registered as his Noble Phantasm, Ashwatthama will never use this. Even in Fate/Grand Order, this Noble Phantasm will never be used.