"We must act swiftly and decisively." —Suncaller Haruh[1]
Inner Will was a free instant self-cast Priest buff that reduces the cost of the priest's instant cast spells by 15% and increases the priest's unmounted movement speed by 10%.
Inner Will and [Inner Fire] are mutually exclusive; only one can be active at a time. For an exploration of the advantages of each, see Inner Fire vs Inner Will.
Although not explicitly stated in the tooltip, Inner Will only affects unmounted movement speed. Mounted speed, flight speed and swim speed are unaffected.
The movement speed bonus from Inner Will does not stack with most other movement speed increasing effects, such as that provided by [Body and Soul].