This is an unofficial ability available via the fan-made mod Unfinished Business for BG2.
It is not part of any official game.
Dark Taint is a Bhaalspawn ability that can be acquired in 
Baldur's Gate II:
Throne of Bhaal (2001)
This icon indicates content from the original Baldur's Gate II: Throne of Bhaal campaign. 
Baldur's Gate II: Enhanced Edition – Shadows of Amn (2013)
This icon indicates content from the Shadows of Amn campaign of the Baldur's Gate II: Enhanced Edition.(ToB) if one has either the Unfinished Business (Baldur's Gate II) or Ascension mods installed.
Mod description[]
| “ | Through this spell, the Bhaalspawn can expel the taint of their evil sire onto a victim, poisoning those who fail their save vs. death. The effects depend upon the caster's level and are detailed below:
7-9th level: 2d8 + 2/round, slow for 12 seconds, save at -2 10-12th level: 3d8 + 3/round, slow for 12 seconds, save at -2 13-14th level: 4d8 + 4/round, slow for 12 seconds, save at -2 15-16th level: 6d8 + 5/round, slow for 24 seconds, save at -2 17-21st level: 8d8 + 6/round, slow for 24 seconds, save at -4 25th level: 8d8 + 6/round, slow for 36 seconds, save at -4, affects group of targets |
” |