Old wards are cut into Vharok's hide, older than the sewers themselves. They don't stop a spell from landing. They make sure the caster feels it too.
A Ward stack (cap 3) and an instant force-damage burst back at the caster. No prior boss here reacts to the ACT of being spell-targeted rather than the damage a spell goes on to deal.
His epic spell answers - a real +6 dodge AC and 10 points of magic damage resistance for the next several rounds - and the stacks reset.
Aventhine Sewers' real ceiling was Vaerith Colm at CR12 (round 8); Vharok is pitched one notch above at CR13, into 2 of the area's remaining Sparky slots.
He is the bestiary's first Runescarred Berserker - another Minotaur cloned from red_minotaur. Aventhine Sewers already has a Dread Necromancer (Vaerith) and an Alienist (Ilqaeth); Vharok is its first pure melee brute.
Real Epic Spell: Mage Armor (id 639/877) - Aventhine Sewers' existing boss (Vaerith) already used Ruin, so Mage Armor went here instead; it also happens to be the one defensive epic spell in this hak, matched to the boss whose whole kit punishes casters rather than melee.