Erjolf is excited about a finding he has discovered in the nearby cave. After some beating around the bush, he agrees to lead you there, as long as you vow to never reveal what it is. Agree and follow him into the cave. Click on the flat rock near Erjolf to cross the river and head deeper inside the cave. Talk to him again and he'll tell you that he thinks he might have spotted a probably-ancient armour, frozen in the ice at the center of the pool, which he thinks he can use to complete his Fremennik Trials.