Spellstrike Pants

The Spellstrike Pants are power caster leg items that boost spell hit rating, spell critical strike rating, and overall magical power.

Source
This item is crafted by Tailors with a skill level of 375.

The pattern drops off Murmur in the Shadow Labyrinth in Auchindoun. The frequency of drop increases dramatically in Heroic Mode. However, it is difficult to say what the actual drop rate is, since it will only show as a drop when there is a Tailor in the group who does not already know the pattern.