This is an unofficial item available via the fan-made mod Unfinished Business for BG2.
It is not part of any official game.
The Web Sack is a wand (WAND14.itm) provided by Pai'Na if the party completes the required elements of the Pai'Na and Spider's Bane quest offered by the Unfinished Business (Baldur's Gate II) mod. The item has a unique red colored icon and appearance.
The wand has expendable charges, and it basically can launch a modified Web spell effect, although it has a greater saving throw penalty. As with most other wands in the game, it can be sold to an applicable merchant, and then repurchased, to "fill-up" its charges again. If all charges are expended by the wand, it will be destroyed.
As with the Web spell, the wand can cause an area of effect web to materialize, potentially exposing those in the area to a webbing animation and potentially inducing a temporary Paralyze effect.