Hunter's Mercy

(Spell Compendium, page 117, DnD 3.5)

Transmutation

Level: ; Ranger 1
Components: S
Casting Time: 1 standard action
Range: Personal
Target: You
Duration: 1 round

Your eyes glow red, but you see the world as normal except when you look at your target. A small glowing dot reveals the creature's weakest point.

This transmutation makes a bow strike true. Your first hit with a bow (not including crossbows) in the next round is automatically a critical hit. If you don't hit in the round following the casting of this spell, the effect is wasted.