As presented in various Mythos tales (most notably The Shadow Over Innsmouth), the “Elder Sign”, perhaps best described as a single line with five shorter lines branching off of it, is a potent ward against the Deep Ones. Whether carved into an oval green soapstone, worn as an amulet or scratched into the dirt, a Deep One seeing this sigil is seemingly unable to harm the bearer or carver.
From an ArM5 mechanics perspective, the simplest representation of the effect of a Deep One or hybrid (regardless of Realm alignment or possession or lack of Magic Might) upon seeing an Elder Sign is to propose the sigil as having the effects of a Traditional Ward Minor Flaw as for Faeries (see Realms of Power: Faerie, page 52, even if the individual Deep One is not aligned to the Faerie Realm. In summary, a Deep One confronted by the symbol cannot directly harm the bearer, cannot regenerate Might points in the presence of the ward and while not compelled to flee, must keep well at bay. Direct contact with the symbol causes the Deep One pain, inflicting damage as if being burnt, but does not destroy the creature instantly.
More potent effects require the equivalent of a Sovereign Ward Major Flaw, which also means the creature cannot indirectly harm the bearer and must flee at the sight of the symbol. Additional application of Flaws such as Baneful Circumstances or variants of Prohibition may also be applicable in selected cases.
In the case of potentially demonically corrupted or Infernal Deep Ones, the Elder Sign may act upon a Demonic Weakness (see Realms of Power, pages 33-34), either acting as an Abhorrent Material/Circumstance, indirectly providing the bearer immunity as a member of a Protected Group (bearers of the Elder Sign) or being a direct Vulnerability of the demonic creature.
A further option for Deep Ones with inherent or learnt magical ability is to possess the Vulnerable to Folk Tradition (sight of the Elder Sign) Minor Flaw detailed in Houses of Hermes: Societates, page 107. This provides characters wearing or making the Elder Sign symbol effective Magic Resistance against the Deep One wizard’s magic and/or the potential to dispel enchantments created by Deep Ones.
Hermetic magi versed in the lore of Deep Ones, whether through Cult of Dagon Lore, (Realm) Lore (Deep Ones) or by discovering this weakness through experience may therefore attempt to utilise this symbol in Hermetic spells and enchanted devices.