When Goblin Arsonist dies, it and Mortuary trigger.
Resolve the Mortuary trigger, putting Goblin Arsonist on top of your library from your graveyard.
Resolve the Goblin Arsonist trigger, dealing 1 damage to an opponent.
Ob Nixilis triggers, putting a +1/+1 counter on itself, and exiling Goblin Arsonist from the top of your library with the ability to play it until your end step.
Cast Goblin Arsonist from exile by paying ({R} magic symbol) .