Area of effect: 7.5m. The caster summons a grease slick for 20 seconds that reduces the movement speed by those standing on it by -50%, as well as causing them to slip unless they pass a physical resistance check against caster's spellpower.
Affected creatures will only slip once (when the spell is cast or when the creature enters the AoE), unless the creature leaves and reenters the AoE.
The effects of Grease Fire and Inferno do not stack.
The effects of multiple overlapping Greases do stack. The slip effects only stack if the areas of effect are slightly staggered. It can take a hostile the full 20s duration to cross two slightly staggered Greases provided it slips both times. Three stacked Greases ensures it takes 20s to traverse from the movement penalty alone.
While a high physical resist will protect against the slip effect some of the time, spell immunity granted by Anti-Magic Ward will do so all of the time. Spell resist from gear does not help prevent slipping. Some creatures are immune to the slip effect as listed in Immunities.
Movement enhancing effects mitigate the movement penalty, while Dispel Magic, Anti-Magic Burst, and Glyph of Neutralization will remove the debuff (along with all your other spell based buffs). Anti-Magic Ward will not prevent the application of the debuff.