The Forgotten Guardian is an armoured skeleton mini-boss guarding the exit of an underground ruin in the side quest Deep Down. It defends well, so go in with your best gear and a patient plan.
Ancient undead guard the old ruins of the valley. The Forgotten Guardian is a tougher foe of the same kind.
Equip your strongest armour and weapon first. Its guard is hard to break, so wait for perfect blocks to open it up. Hit once, then go straight back to defending. Watch the swords: a glow means the unblockable twin strike is coming.
Beating it clears the exit from the ruin and drops the Undead Warrior's sword. Deep Down itself pays 250 XP and 25 denarii when you report back to Gavril.