the weapon which was used to slay ghatotkach in mahabharatha

Ghatotkach, Bhima's son, was killed by Karna using the Amogh Shakti (also spelled Amoghsakhti or Amogha), a divine spear/weapon given to Karna by Indra. This weapon could be used only once and was saved for Arjuna, but Karna had to use it on Ghatotkach when the latter was devastating the Kaurava army at night. Ghatotkach's death was a turning point in the war.