Abubakar Shekau, the leader of Nigeria's Boko Haram extremist group, was once rejected by ISIS for being too radical.
Shekau, who reportedly killed himself during a gunfight, according to West Africa's ISIS affiliate, took the reins of Boko Haram in 2010 after its founder Mohammed Yusuf died in police custody.
The terrorist, whose age was unknown, leapt to international notoriety in 2014 with the abduction of 276 schoolgirls in the remote town of Chibok, sparking the #BringBackOurGirls movement.
Soon after, the UN Security Council sanctioned the group and the United States declared Shekau a "global terrorist".
Born in Nigeria's northern Yobe state, Shekau was an ethnic Kanuri whose parents were poor farmers who immigrated from neighbouring Niger.
After embracing hardline religious ideology, the young Shekau became a local preacher. In 1990, he moved to the Borno state capital Maiduguri.
"He was an easy-going fellow who would exchange banter with people in the neighbourhood. He was popular... as a local theology student," said Grema Kawudima, from the Mafoni area of the city, in a September 2012 interview.
According to another local, Butari Gwoni, Shekau married a teacher's daughter, but she died in childbirth, setting the stage for his drift to radical ideology, according to some.
In a widely cited 2012 quote from one of his first propaganda videos, Shekau said: "I enjoy killing... the way I enjoy slaughtering chickens and rams."
Shekau began his attempt at becoming a theologian when he enrolled at the Borno state College of Legal and Islamic Studies.
There he met Mamman Nur, who would later mastermind the August 2011 bombing of the UN offices in the capital Abuja, which killed 26 people. Nur introduced Shekau to Yusuf, Boko Haram's founder.
"He [Shekau] was a simple, carefree fellow at the beginning," said Kayam Bulama, a student at the same time.
"But soon after Mamman Nur linked him up with Mohammed Yusuf, he began to be abrasive and radical, shunning other students and keeping company of his fellow sect members."
Yusuf was killed in 2009 during a military crackdown on the group that left about 800 people dead. The group went underground briefly afterwards, until Shekau took control.
Under Shekau's leadership, Boko Haram turned large swathes of Nigeria's north-east into a no-go territory, proclaiming a "caliphate" in the Borno town of Gwoza in 2014.
A major offensive in 2015 by Nigerian troops backed by soldiers from Cameroon, Chad and Niger drove the terrorists from most of the area that they had once controlled.
Despite ongoing military operations against Boko Haram, the group has shown resilience and continues to stage bold attacks.
In December, it claimed responsibility for slaughtering 76 farmers outside Maiduguri.
Last month, Boko Haram militants in several trucks fitted with machine guns and on motorcycles stormed parts of Maiduguri, before they were repelled by security forces.
In his propaganda, Shekau often appears unhinged, with wild-eyed rantings that celebrate acts of horrific violence.
But many disagree with the view that he suffered from a mental illness.
"He is obviously clever for being able to be a leader for so long," Jacob Zenn, a researcher at the Washington-based Jamestown Foundation, told AFP.
Jacob Zenn, researcher at Jamestown Foundation
"He is mercurial, subject to quick temper, extreme, and stubborn," said Mr Zenn. For more than 10 years, "he has not changed, he is consistent, unwilling to compromise".
His unwillingness to comprise is what led to a split within Boko Haram.
Shekau encouraged brutal tactics, including the use of women and children suicide bombers, something other militant groups disapproved of, although the same groups, linked to ISIS, frequently advocated suicide attacks using adult bombers.
"He considered Muslim civilians who were not loyal to be legitimate targets," Crisis Group researcher Vincent Foucher wrote in a report.
"He also antagonised many other commanders," he added, "with what they saw as his hoarding of cash and weapons and his refusal to share this bounty with those he did not like or trust."
In 2016, senior Boko Haram commanders who disagreed with Shekau's tactics deserted the group's stronghold in Sambisa forest and headed west towards the Alagarno forest.
A few months later, the leader of their faction, Habib Yusuf, a son of Mohamed Yusuf, was officially recognised leader of ISIS' West African affiliate.
ISIS in West Africa has since gone through several internal changes, but is now the dominant threat for the Nigerian army.
In audio obtained by AFP from the same source who conveyed previous messages from the group, a voice resembling that of the West African ISIS leader Abu Musab Al Barnawi said Shekau died after being hunted down for five days by its fighters.
"Shekau preferred to be humiliated in the hereafter to getting humiliated on Earth. He killed himself instantly by detonating an explosive," said the voice speaking in the Kanuri language.
Boko Haram has not yet officially commented on the death of their leader while the Nigerian army said it was investigating the claim.
