At 15% he can cast an instant Aoe knockback and also afflict a 15 sec Debuff with
"reduce all stats reduced by 100. 740 to 860 Shadow damage inflicted every 5 sec."
It is possible to LoS with the tree close behind him and AoE range is around 20yd i believe, but he can cast it again shortly after the 15sec is up so watch out if playing HC.
