Sporeback Troll dies, its persist ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it due to its graft ability.
When
Sporeback Troll enters,
Blessed Sanctuary triggers, creating a creature token.
When the creature token enters,
Sporeback Troll triggers, moving a +1/+1 counter from it to the creature token.
Sporeback Troll dies as a state-based action due to having 0 toughness.