The Dark Lord always fought like a threat. He didn't always look like one — walk into the nave before this round and you'd find a very dangerous swordlord standing very plainly in a very dark room. The fight was there. The dread wasn't, not the way it should be for the thing sitting at the end of the whole cathedral. That gap just closed.
The moment he notices you, he dresses for it. A void aura wraps him, cold and creeping, the kind of effect that makes the air around him look wrong before he's swung once. His blade picks up its own glow, so every read on his attacks now comes with a light cue as well as a telegraph — watch the sword, not just the stance. None of it is decorative flourish that vanishes after one hit; it rides with him for the whole fight and only breaks when he does, dismissed the instant he falls.
The court he calls answers in kind. When he raises his horrors partway through the fight, they no longer simply appear — they climb up out of proper void portals, tears in the floor that open, disgorge something that used to be a knight, and seal behind it. It's a small change with outsized weight: a monster that steps out of a hole in reality reads as considerably more wrong than one that just spawns. And when the room-wide Purge comes, or when he finally falls, a dark burst answers it — a beat of violence the eye can't miss, marking the moment the fight turns.
None of this touches the numbers. The dodge windows, the phase gates, the fairness of every read are exactly what they were — this is a costume change for a fight whose mechanics were already earning their keep. But a boss room is a promise, and the promise was always that this thing sitting at the heart of the cathedral was worth the whole walk there to face. Now, for the first time, he looks like it the second he notices you're in the room.