What is the Santana song "Incident at Neshabur" about?

They are probably referring to the city Nishapur in Razavi Khorasan province in northeastern Iran.

https://en.wikipedia.org/wiki/Neshabur
https://en.wikipedia.org/wiki/Abraxas_%28album%29

This instrumental track is on a 1970 album from Santana called 'Abraxas'. With no lyrics, it is hard to say what it is 'about'. Presumably something happened to someone while they were in Nishapur (Neysh?b?r, Neshabur) in northern Iran. The track was written by Carlos Santana & Alberto
Gianquinto.

https://en.wikipedia.org/wiki/Abraxas_%28album%29

It's apparently about a revelotionary in Haiti at the turn of the 19th century.

This is a direct quote from Carlos Santana:

Neshabur is where the army of Toussaint Louverture - who was a black revolutionary - defeated Napoleon in Haiti. So that's what it's about.

https://en.wikipedia.org/wiki/Incident_at_Neshabur

Except:
Toussaint Louverture was betrayed and captured, finally dying in France.
The final victory was won by his second in command (at the Battle of Vertieres) shortly after that.
And good luck finding anywhere on the island with a name like Neshabur.

Maybe Santana was kidding ...