Ghouls and ghasts are flesh-eaters; specters, wights and wraiths are life-drainers. Driven by malice, envy and despair, they compulsively consume the vitality they themselves can never possess again. They all possess darkvision and shun the sunlight, so they’re encountered only at night; in some shuttered, haunted locale; or underground.
Specters are incorporeal: they have no material existence and can pass through solid objects and other creatures, although they can be harmed by stopping inside one. They have a very fast flying movement speed of 50 feet per turn. Their only exceptional ability scores are a Dexterity of 14 and a Strength of 1. They’re immune to most debilitating conditions, although they can be blinded, deafened, frightened, incapacitated or stunned. They’re immune to necrotic and poison damage and resistant to other forms of damage, except for radiant damage and physical damage from magical weapons. Being of normal humanoid intelligence and Wisdom, they have the sense to back off from an opponent that starts inflicting these types of damage upon it.
A specter might be expected to identify and zero in on weaker victims, and if it were an evolved creature, it might act that way. But specters operate are driven by their compulsion, not by adaptive survival instincts. When they sense the presence of a living victim, they don’t evaluate how easy or hard it will be to devitalize. They just wanna kill everybody, and it doesn’t matter who’s first.