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

Word for word it is Every {count}th critical strike envenoms 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 Venom behind, which makes it Poison build material.

6 bases carry it, and 1 Unique is built on those: The Overlord's Retinue.