Battlemage is essentially a Mage-Juggernaut; great sustained damage, immobile and having defensive qualities.
Imo Morde's neither a True Juggernaut nor a Battlemage but a Hybrimd of the two.
Thematically he's an undead Conqueror-King that taps both into the forbidden dark arts of necromancy and his very own unholy physical strength.
He's the quintessential Dark Lord character similar to Sauron who increases his already remarkable physical strength with magical energies as shown by his choice of weapon; the ensorcelled Mace Nightfall.
Him being Melee puts him closer to Juggernaut so I'd say Juggernaut with Battle-Mage qualities would be the appropriate classification.