War God’s Greatsword

Weapon (greatsword), very rare (requires attunement)

A war god’s greatsword is an adamantine greatsword that gains additional power as its wielder defeats worthy foes. Whenever you kill a foe with a CR at least 1 higher than your character level, the sword gains one charge. The sword can hold up to 5 charges. You can unleash any number of charges as an action to frighten all enemies within a 30-foot radius. Those that fail a DC 14 Wisdom saving throw are frightened for 1d4+1 rounds; those that succeed are frightened for only 1 round instead. For each additional charge the wielder expends, the save DC is increased by 1 and the duration of the frightened effect on a failed save is increased by 1 round.

The sword gains power from its connection to the prowess of its current wielder. Whenever a new creature wields the sword, it loses all of its charges.

A creature must truly die for the sword to gain power—as such, it does not gain power from slaying summoned creatures. Except in extraordinary circumstances decided by the GM, the sword cannot gain power from a particular creature more than once.