Haruman

In 1354 DR, Haruman was one of the people who chose to accompany Zariel into Avernus, in the event known as the Ride, as one of her generals.
Upon arrival in Avernus, Haruman and his fellow general Olanthius refused to leave Zariel's side and accompanied her to Nessus, witnessing there Asmodeus offering Zariel rulership of Avernus.
Haruman chose to follow Zariel into damnation, and was transformed into a narzugon devil
"Behold! Zariel’s dog crucified those who betrayed his mistress in battle.
— The Cartographer"