Night's Silence is a craftable set. The 5-piece set bonus ignores the Movement Speed penalty of Sneak.
Night's Silence is part of the base game. The crafting station are located in Auridon (Hightide Keep), Glenumbra (Mesanthano's Tower) and Stonefalls (Steamfont Cavern). You can craft any armor, weapon or jewelry of this set in any motif style you know, as long as you have 2 traits researched (per item). You can trade crafted pieces to other players. Each set item becomes bound on equip.