Complex sentence: A student leader who went on to found the Black Consciousness Movement (BCM) which empowered and mobilized much of South Africa's urban black population, he died in police custody, and has been called a martyr of the anti-apartheid movement.
Split into simpler sentences: A student leader, he later founded the Black Consciousness Movement (BCM) which would empower and mobilize much of the urban black population. Since his death in police custody, he has been called a martyr of the anti-apartheid movement.