Assassin's Guile is a craftable set. It increases the duration of your alchemical poisons by 4 seconds.
This set is used in niche builds in Cyrodiil PvP with (for example) cost poisons; their normal duration is 6 seconds, but with this set you can boost it up to 10 seconds depending on what poison you use for your weapons. Keep in mind that poisons suppress your weapon enchantment, so choose whichever works best for you.
The Assassin's Guile set is part of the Morrowind Chapter. The crafting station is located in Vvardenfell (Marandus). You can craft any armor, weapon or jewelry with this set in any motif style you know, as long as you have 3 traits researched (per item). Each crafted set piece can be traded to other players; however, the pieces become bound on equip.
ESO
Sets