Essence eye is a skill to display how long it will be until a spell you’re casting is complete. At 100% skill it should always give casting information every heartbeat. A cheap and useful utility skill.

The ingame help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: None.
It uses no endurance points.
The essence eye skill provides a graphical feedback of
the spell you are currently casting.  It will display 
the number of rounds left until the spell is casting
is done.  The higher your percent in essence eye the 
more often the output is updated.  

Example of essence eye output:

Money is power: ############

In this case, money is power is the spell you are 
currently casting, and it will be complete
in 12 rounds.  Each '#' is 1 round.