Who obtained boon to kill bheeshma

Amba, the eldest princess of Kashi, obtained a boon from Lord Shiva to be the cause of Bhishma's death in her next life after Bhishma rejected her marriage proposal and abducted her sisters. This boon eventually led to her reincarnation as Shikhandi, who played a crucial role in Bhishma's death during the Kurukshetra war. Amba's vow of vengeance and Shiva's blessing directly enabled Bhishma's ultimate fate.