Well now that he's got a set of animations (idle, walk, pain, block, attack, spin attack, summon crows, riding attack, death) this opens up some options:
a) Allow the Assassin to summon him - uses full mana bar, and the Death Dealer rides in and for 30-45 secs will attack all enemies on screen
b) Have a quest to find the Skull of the Death Dealer, anyone who gets it can use it like option A.
c) Have the Death Dealer be a boss that you have to fight.
d) Have the Death Dealer be the final boss of the Hippodrome gladiator arena
I'm leaning towards A, but would love to get some input on this.