Stolen from the Warhammer 40 Role Playing Game. Sounded appropriate for an aberration/mind flayer focused campaign I am running – Thoughts of Darkness.

The Maul of the Gatekeeper

Weapon (maul), legendary (requires attunement by a creature of lawful alignment)

The sacred relic weapon carried by the Watch-Captain is an artefact held in great reverence by the Deathwatch and the Storm Wardens Chapters. An ornate mattock, without a single blemish, the Maul of the Gatekeeper is said to be the bane of all xenos and to date back to the founding of the Storm Wardens Chapter. Due to the pall of mystery surrounding the Chapter and the mysterious Nemesis Incident, no more information than this is known, though numerous legends are told of the weapon’s capabilities.

In battle, the Maul of the Gatekeeper is treated as a +3 magic maul that does thunder damage instead of bludgeoning damage.  However, Player Characters may spend Fate Points to augment the damage it inflicts when fighting non-humans of any type. This augmentation is declared before damage is rolled, and each Fate Point spent adds an additional d6 to the weapon’s damage for that attack only. When fighting aberrations, the result of the extra d6 is doubled, and any creature dominated, charmed or in any way a mind-slave of an aberration struck by the weapon (even if no Fate Points are spent) has its enslavement immediately cancelled and the creature is immune to enchantment effects for the next 24 hours. Given the nature of the Maul of the Gatekeeper, the players should not be informed of its effects unless one of them actually wields it against a xenos (aberrant) foe.

Proficiency with a maul allows you to add your proficiency bonus to the attack roll for any attack you make with it.