http://wowpedia.org/Patch_1.3.0
www.wowwiki.com/Patch_1.3.0
Nightfall - Fixed a bug where the Shadow Trance effect was lost without benefit. Previously, the talent had a 3%/4% chance of triggering. It now has a 2%/4% chance of triggering, and the tooltips have been updated to accurately reflect this change.
В общем во всех источниках где я смог глянуть шанс, он почему то везде 4%...

- - - Updated - - -

http://www.wowwiki.com/Nightfall_(wa...?oldid=2366148
Тут так же 2 и 4%.