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