Dead things clearly transcend beyond this plane as their corpse disappears. Or so the priests say.
The catacombs are places of prayer and worship, where you request favors from those who have passed beyond and become unto gods.
The sick and dying are brought to such halls, sometimes to pray for freedom from pain, sometimes to follow their ancestors. Priests have arranged "magical energy"-capturing devices in the catacombs to capture and channel the energy of the dead, fueling the miracles which are used to maintain the theocracy or benefit the community.
What more, the means for capturing the life energy actually does create a form of awareness in the weave of mana. Memories, especially memories at the moment of death, collect. Prayers, repeated over generations, shape.
The processing of such memories and awareness takes time and repetition, so ancestral tombs full of the same family can generate a fondness and connection with a living line over the centuries, but a single death won't fuel it. Repeated prayers and supplication guide it. Love and admiration keep it from souring.
Catacombs that are abandoned go stir crazy. Such abandoned catacombs are "haunted", and the aware mana weave can even animate and manifest physically when disturbed.
To provide for interaction, catacombs have statues (of the dead, and the guardians of the dead) for the supplicants to interact with, and the mana weave to animate when particularly driven. These animated statues can even defend the Catacomb against defilers. In abandoned Catacombs, the animated statues are going to be hostile to anyone who doesn't carry the cultural markers and behavior of those who "should" be there.
Catacomb robbers thus can learn the cultural signifiers of the dead culture, carrying the holy symbols and learning the prayers and behavior to ward off the undead horrors.
Societies that engage in mass sacrifice to fuel their magics generate a local mana weave that is full of hate and fear and pain. These are scarier places than simply ancient catacombs. The structure tends to be different, where the tormented spirits are forced to do useful work, but the structures will have broken down over the ages leaving you with a hell pit of hostile spirits.
One effective way at least some cultures use to contain the sacrificial spirits is to surround it with friendly spirits. So sometimes you are exploring a merely abandoned catacomb, and you enter the sacrificial region and all hell breaks loose.