Regenerate target creature. Draw a card. (The next time the creature would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)
Spell mastery — If there are two or more instant and/or sorcery cards in your graveyard, also regenerate each other creature you control.
6/22/2015 You draw a card as Dark Dabbling resolves, not as the creature actually regenerates.
6/22/2015 Whether the spell mastery applies or not, Dark Dabbling targets only one creature. If that creature becomes an illegal target by the time Dark Dabbling tries to resolve, Dark Dabbling won’t resolve and none of its effects will happen. No creature will regenerate and you won’t draw a card.
6/22/2015 Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.