Question Summary:
I would like details about Hadrat Ali’s (radiyallahu anhu) demise?

Question Detail:

1)Who martyred Hadrat Ali (radiyallahu anhu) and why and when was he martyred?

2)What was the date when Hadrat Ali (radiyallahu anhu) was martyred?
3)Where is the grave of Hazrat Ali (radiyallahu anhu)?
 

Answer :

1.) Amir-ul-Mu'minin Ali (radiyallahu anhu) was martyred by Abdur-Rahman ibn Muljam a member of the khawārij sect who was the most wretched person of the later times. 
 
In an authentic hadīth, Rasulullah (sallallahu alayhi wa sallam) asked Ali (radiyallahu anhu): “Who is the most wretched of the earlier times?” Ali (radiyallahu anhu) replied, “The one who hamstrung the she-camel.”  He (sallallahu alayhi wa sallam) asked, “Who is the most wretched of the later times?” Ali (radiyallahu anhu) replied, “Allah and His Messenger know better,” upon which the Prophet (sallallahu alayhi wa sallam) replied, “Your killer.” [i] [ii]
 
Abdur Rahman ibn Muljam's reason for killing Ali was that he wanted to take revenge on Ali (radiyallahu anhu) for putting down the khawārij rebellion at Al-Nahrawā<span