Chthonic Fissure deals fire and necrotic damage. But which penetration will be better for this? fire or necro? or both? I don’t respect fissure to poison…
By default, both are equally as effective so unless you’re adding flat necrotic spell damage such as from the Profane wand, they’ll always be as effective as each other. Necrotic penetration will only affect the necrotic damage & fire penetration will only affect the fire damage.