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While wielding this weapon, your character may suffer 2 strain and make a Melee (Light) check as a ranged combat check against a target within medium range, using your character’s Willpower instead of Brawn. If the attack is successful, add your character’s Willpower to the damage, instead of their Brawn.

Thoughts? Does it make sense, or too clunky?

I would rewrite it as a secondary attack:

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Spend two strain and make attack with the following profile:

(+x; Range: Medium; Ranged (Light); crit: x; qualities: Use Willpower for both the Ranged (Light) check and damage bonus)

Though the wording could still use some cleaning.

What do you want with this ability?

It's sort of a magic sword beam, native to the weapon itself, which is why it uses Willpower and costs 2 strain like spells do.

Well, it could be a Weapon Quality: Energy Strike .

The character can use the melee weapon to do a ranged attack. He will lose strain equal the ranged difficult and he can't lose more strain in this way than his Willpower. With 2 Advantages, he avoid the strain cost.

No. I typically avoid adding new weapon qualities if at all possible. This is a special case anyway, as it's supposed to be in the same category as the magic items from RoT.

Oh, well, I don't have RoT so I don't know how magic itens work there ?

I see how the mechanic works and I think regardless of how you word it, it's intuitive and works within the framework of the way the game works. Basically, once a player makes the roll 2 or 3 times, it should click and they'll be able to handle it.

The only thing that I would request is that you name the sword the, "Pewpew Stabby Stick"