Since the patch lowered Thorgrim Beastslasher to level 14 and took many elites of low level bosses (including skull crack) i haven't been able to find the skill...
it belongs to hail blackice in the spearhead peaks, south of granite citadel. he is next to rune ethercrash. you need to have someone cast in front of him so he would use it; it would take a long long time with just henchies. -aries
