When used with an Orb of AnnulmentOrb of AnnulmentStack Size: 20Removes a random modifier from an itemRight click this item then left click on a magic or rare item to apply it., it will remove the modifier with the lowest item level requirement on an item. Note that this is unrelated to the Tier of the modifier.
For example, if you have a quiver with the following modifiers:
T3 Adds # to # Fire damage to Attacks (ilvl 20)
T2 +# Level to all Projectile Skills (ilvl 41)
T4 #% increased Attack Speed (ilvl 60)
T8 +# to Dexterity (ilvl 74)
The Fire damage modifier will always be removed. Item level requirements can be found on third party websites such as Craft of Exile or PoE2DB.
Item acquisition
Omen of Whittling has restrictions on where or how it can drop.