Like the legion idea, but would like it somewhat expanded.
My idea goes like this.
While walking the castle, Gabula looks up, on the ceiling sees the bulging lower mass of legion, like a great ass sitting on a toilet seen from underneath. It is a creature of what must be grotesque proportions, covered in the bodies of the condemned dead, allowed to exist on the mortal plane within the castle walls, for here one may say they walk in the land of the dead and of the living.
Suddenly, an eye appears in the middle of the encircled mass. It stares as Gabula, and he stares back. Gabula, eyes locked on creature, kneels slowly. Then, placing his hand on the ground, power builds up in a circle beneath him. With a jolt he's flying strait up, willing himself faster and faster. Shooting strait up like an arrow, trailed by dark power, he unsheathes his sword. Aiming it strait up, he becomes a spear, blasting through the creatures eye, burning through at least sixty feet of corpses surrounded by white hot flames.
Finally breaking through, he lands on the inside of the creature. In the middle of this hallow space is a great and bulging pulsating brain with multiple eyes. Extending to the brain are massive tenticles, all leading to the outer shell of bodies, extending into masses of veins spreading out everywhere. Taking this all in, Gabula feels a touch, and just manages to keep from being bitten and devoured by a decaying zombie.
Anyhoo, you get the point. instead of fighting legion on the outside, he fight the creature on it's inside, running around cutting its tentacles off one by one, while being chased by naked zombies. You could even have fissures and inner tentacles that squirt acid at him in a circler pattern, which he would likewise have to avoid by following it's movements like in its 2D appearances. Eventually, cutting enough tentacles, you get the eyebrain to fall, and then you have to kill that. They should have it attack you like a giant Beholder. Would be cool.
We've fought legion the old way too many times to count. Time to mix it up a bit.