The first symbiote, All-Black was manifested from the shadow of the evil deity Knull and tempered using the divine power of a slain Celestial. It possesses a mind of its own, but even so, it is loyal to you. Though it may want to kill gods.
The Necrosword can harvest power from the dead, augment your own divinity, use dark energy, and slay gods. The All-Black grants you the power of Knull.
Your Necrosword lacks the conventional weaknesses to fire and sounds like normal Symbiotes do. But even so, it is afraid of intense heat and sonic vibrations.