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Very rare is about right

Stun is a powerful condition to inflict.

Very Rare is the lowest I would suggest, particularly with the multiple resistances feature. Note that rings of resistance to a single element are rare items. (Ring of fire resistance, etc).

While comparing it to other sentient items like Blackrazor, Whelm and Wave is useful as a point of departure, those three stand out as well outside the norm for magic weapons.

Very rare is about right

Stun is a powerful condition to inflict, when it lands¹.

Very Rare is the lowest I would suggest, particularly with the multiple resistances feature. Note that rings of resistance to a single element are rare items. (Ring of fire resistance, etc).

While comparing it to other sentient items like Blackrazor, Whelm and Wave is useful as a point of departure, those three stand out as well outside the norm for magic weapons.

¹ _{Stun save DC of 14 (CON) is going to be more effective on low CR minions than on High CR opponents as you get into the upper tiers of the game, with a few exceptions like humanoid spell caster NPCs. This footnote is intended to capture the point in the comments by SeriousBri and by Nobody The Hobgoblin.}

Very rare at the least

Stun is a powerful condition to inflict.

Very Rare is the lowest I would suggest, particularly with the multiple resistances feature. Note that rings of resistance to a single element are rare items. (Ring of fire resistance, etc).

While comparing it to other sentient items like Blackrazor, Whelm and Wave is useful as a point of departure, those three stand out as well outside the norm for magic weapons.

Very rare is about right

Stun is a powerful condition to inflict.

Very Rare is the lowest I would suggest, particularly with the multiple resistances feature. Note that rings of resistance to a single element are rare items. (Ring of fire resistance, etc).

While comparing it to other sentient items like Blackrazor, Whelm and Wave is useful as a point of departure, those three stand out as well outside the norm for magic weapons.

Very rare at the least

Stun is very powerful as a condition.

Very Rare is the lowest I would suggest, particularly with the multiple resistances feature. Note that rings of resistance to a single element are rare items. (Ring of fire resistance, etc).

While comparing it to other sentient items like Blackrazor, Whelm and Wave is useful as a point of departure, those three stand out as well outside the norm for magic weapons.

Very rare at the least

Stun is a powerful condition to inflict.

Very Rare is the lowest I would suggest, particularly with the multiple resistances feature. Note that rings of resistance to a single element are rare items. (Ring of fire resistance, etc).

While comparing it to other sentient items like Blackrazor, Whelm and Wave is useful as a point of departure, those three stand out as well outside the norm for magic weapons.