Storm's Eye fires every so many crits. On Faded Starcaller Relic it reads: Every 5th critical strike shocks the target for 60% of the hit.

Word for word it is Every {count}th critical strike shocks the target for {value}% of the hit, and the number in it grows with the gear: 60 to start, 4 more for every item level, and it only goes off every 5 times you set it up.

It leaves Shock behind, which makes it Storm build material.

6 bases carry it, and 2 Uniques are built on those: A Patch of Night and The Sky's Own Star.