Quote from WrathofanimaSo, essentially, yes; a creature or a spell can be a source of damage, but it is not limited to just those.
Not quite. Whatever the effect says deals the damage, as in "Lightning Bolt deals...", is the source of that damage. Spells can be sources of damage, but not abilities. In combatdamage, the creature delaing the damage is the source, not "combat damage" itself.