Alright, so you're asking about symbols for dark gods. It's a huge topic across different cultures, and honestly, 'dark' is a pretty subjective filter—sometimes it's about destruction, sometimes chaos, the underworld, or just forces outside the benevolent order.
For underworld deities, you've got the obvious ones like Hades' bident (that two-pronged fork) or his Helm of Darkness, which is more an artifact but still a symbol. Cerberus, the three-headed hound, is another strong visual tied to him. Over in Egypt, Osiris might be a lord of the dead, but he's not exactly 'dark'; more ambiguous figures like Set, the god of chaos and deserts, have animals like the Set animal (a weird composite creature) and the color red as chaotic symbols.
Then there are entities linked to destruction and time. Kali Ma from Hinduism, with her necklace of skulls, skirt of severed arms, and lolling tongue, is a powerhouse symbol of time, destruction, and rebirth—often terrifying but also liberating. The Norse have Hel, whose name gives us the place, and her appearance is half-living, half-corpse, which is a symbol in itself. Loki's association with chaos and betrayal isn't always a single object, but snakes and trickster animals come to mind.
Some symbols get more abstract. The ouroboros—the snake eating its own tail—is linked to cycles, including destructive ones, and appears in Gnostic and alchemical texts with demiurge figures. Pentagrams or inverted crosses are more modern associations, often linked to fictional or Satanic imagery, but they don't have deep roots in most ancient mythologies as direct god symbols. It's fascinating how many of these 'dark' symbols aren't purely evil; they represent necessary, if frightening, parts of existence like death, change, or primal chaos.