Sanity's Helm

The most feared opponent of any Vorstborn dwarf, hoping to survive the harsh environs of the Underdark, is the mindflayer. Though the great gates of Vortshold can withstand armies of undead, of Duergar, of Drow or even the fearsome rays of a Beholder, it is the insidious tentacles of the Illithid that each dwarf prays never to face in battle.
Not much is known of Mavis Mantine. What little records remain place her as only a distant relation to the Mantines on the Vorsthold War Council, and as a Deepfast commander long ago on the frontlines of the war with the Underdark.
The story goes that she led an assault on a mindflayer nesting grounds, piercing through their ranks and destroying the rancid pools where their tadpole offspring are kept waiting for hosts to burrow into. So successful was she that Mavis made it all the way to the Elder Brain - the giant pile of brain matter and tentacle that bound the whole colony together - the hive mind. After a fierce battle, slew it and the whole colony was annihilated. In the wreckage of that place though, she discovered something - this colony had actually been at war with another mindflayer colony deeper still into the depths of the earth. What sealed Mavis into legend was her discovery of a tool they used in this intra-Illithid conflict - a helmet, charged with psionic energies to keep their mind shielded from others of their kind.
For almost a year she worked on it back in the forges of Deepfast, adjusting it bit by bit, augmenting its magic with those of dwarven design, until finally it was done - Sanity's Helm. Wearing it kept the wearer completely protected from any of the psychic manifestations and manipulations of the mindflayers.
She could never replicate the feat - the Illithid magics could not be done again on another helm - but with this on her head she became a menace to her chosen foe, hunting them down over and over again to keep her people safe as she decimated the local Illithid population.
The stories vary on what happened next - the hopeful warriors say she fought until she could no more and finally hung up her helmet somewhere safe, guarded by the War Council until a great need arises again; those who like horror stories say Mavis kept finding more artifacts of the mindflayers and turning them to her purposes until she became so enveloped by them that she forgot she hadn't been a mindflayer all along and turned on her people; the pragmatists say that her story ended like all stories with mindflayers end - with a betrayal from a companion removing her helmet in battle so that she could finally be overwhelmed, Sanity's Helm reclaimed by its creators once again.
What is certain is that none have seen this fabulous helmet again, if it ever existed at all.