- Pick a direction to aim;
- Deduct 20 arrows per attack.
Treat like a lightning bolt that doesn't rebound. Everything in that direction, friend or foe, takes damage equal to 1d6 + target's AC. If they make a save, they take half damage. If the damage seems low for that many arrows, remember: you're essentially creating a "zone of death" without the need to make an attack roll.
This is not a feat. It's just a quick ruling on how to implement something any fighter might try.