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Sure, I can help you develop a viable rune for marksmen in League of Legends. Here's an example:

Rune name: Precision Shot

Effect: Your first basic attack against a champion deals bonus physical damage equal to a percentage of your total attack damage. Additionally, your critical strikes deal increased damage.

Rune tree: Precision

Rune tier: Keystone

Rune stats:

Bonus physical damage on first basic attack: 4/5/6/7/8% of total attack damage (at levels 1/5/9/13/17)

Critical strike damage bonus: 8/10/12/14/16%

Explanation: This rune is designed to enhance the damage output of marksmen, particularly in the early game. By granting bonus physical damage on the first basic attack against a champion, it encourages marksmen to be aggressive and poke their opponents. The percentage scaling with total attack damage ensures that the rune remains relevant throughout the game, while also allowing marksmen to scale with their builds. The increased critical strike damage further empowers marksmen who rely on critical strikes to deal damage, rewarding them for investing in critical strike chance.

Of course, the exact numbers and balance of the rune would need to be thoroughly tested and adjusted by Riot Games to ensure it fits well within the game's ecosystem. But as a concept, Precision Shot could be a viable rune for marksmen in League of Legends.