Is this ability considered magical damage?
There is no such thing in GD, just different types of damage some of which goes straight throug mundane armour and gets resisted by corresponding resistance.
In short yes, DS does indeed “magical damage” and also sucks life makin it one of the best “leveling” skills.
Too bad the skill doesn’t have any cool modifiers or transmuters.
So in other words, does spirit increase its base damage?
Sorry, I’m new to this game
Yeah, but as a new player havin your first toon I advice to put your points into Physique for Life and Defensive Ability, investing into Spirit is kinda pointless unless you need to equip gear requiring it.
Wildheart Vanguard buffs it some. There is also an MI that modifies it I think.
Cunning increases Physical, Pierce and Bleeding damage (including DoT equivalents).
Spirit increases Fire, Cold, Lightning, Acid, Vitality, Aether and Chaos damage (including DoT equivalents).
Devouring Swarm does Vitality/Bleedibg I believe? Both attributes would I crease it’s damage.
Devouring Swarm is not meant to be a damage skill is why. Even if it is solid for levelling at the start, it tapers off later on while other levelling skills stay strong.
To my knowledge, Devouring Swarm is a debuff that also happens to do damage. I’ve heard that it can never miss nor crit as it is not checked by the PtH formula.
What’s the name of Bleed DoT then? 
Just kidding. 
moar bleed.
But then again “One-point-wonder” Grasping Vines has a modifier.
DS is strong with it’s RR, so buffing it would imo make it maybe too good.
That used to be the case, many, many versions ago. Devouring Swarm can crit now. I think it changed when DoT gained the ability to crit.
Edit: but I’m pretty sure that it still cannot miss.
Devouring Swarm is still a debuff that deals damage.