Maulana Sami-ul-Haq, known as the "Father of the Taliban" for having taught leaders of the terror group, was found killed on Friday in a Pakistani city, a relative and his deputy said.
Unknown attackers killed the cleric, who runs an Islamic seminary in northwestern Pakistan, his deputy Yousaf Shah said.
There were conflicting reports of exactly how he was killed and why his bodyguard and driver were apparently not there to defend him at the time of the attack.
Shah initially said that Haq had been shot dead.
Haq's nephew Mohammad Bilal told Reuters that his uncle was found with stabbing and gunshot wounds in a house he owns in an upscale area on Islamabad's outskirts. "When the assailants entered his house ... They first started hitting Mullah Sami ul-Haq with knives and daggers and then shot him dead," he said.
Further details remained unclear.
Haq has run the Darul Uloom Haqqania seminary in Pakistan's Khyber-Pakhtunkhwa province, near the Afghanistan border, for decades.
One of his students from the 1980s, known later as Mullah Mohammad Omar, went along with classmates to Afghanistan to join mujahideen groups fighting against the Soviet occupation of the country.
Mullah Omar went on to found the Taliban, which seized power in Afghanistan in 1996 after years of chaos and civil war following the Soviet military's withdrawal.
The ultra-conservative Taliban imposed an extreme version of Islamic sharia law on Afghanistan that included forbidding women to leave home without a male relative, imposing minimum lengths on men's beards and banning sports, radio and television.
Haq's seminary has continued to thrive in Pakistan, including being allocated funding in provincial government budgets.
Pakistan's Interior Ministry confirmed Haq's death in a statement on Friday evening, and expressed condolences.