Eagle Eye is a skill in the Hell Fire skill tree of the Mage class.

Skill Description: Your spell range is extended for 5M for 4Minutes 30seconds.
Required job: First Job
Required points: 5 points in Hell Fire
Skill type: Support Skill
Weapon Required: Staff
Attribute: Darkness
Cool time: 300 seconds
MP Usage: 8 MP
Target: Self

Handy, but not required. It buys you time for your DoT effects to build up, or your cool downs to end.

One nice thing is that when you lose your 5' range you notice right away, which also means you need to rebuff your Incomplete Witcheryer