Improved Enslave Demon was a warlock talent in the Demonology tree, you needed to spend 27 points in Demonology to max it.
This was a potentially-powerful talent to have, depending on style of play. If you enslave demons, such as the summonable Infernal and Doomguard, often, or appreciated the added utility option, then this was well worth taking. If not, then this talent was not a good option.
Patch 1.5.0 (2005-06-07): In addition to reducing the attack speed and casting speed penalty of enslaved demons, it now reduces the resist chance of Enslave Demon by 2/4/6/8/10%.