You go first, the ghouls will spend the rest of this round digging their way out, so you have 2 rounds of actions you can now post.

(A few questions? Could Bernard see the ghouls? If so, could he hit all 4 with something like a Burning Hands? If not, could a cover-ignoring Sacred Flame do the trick?)

The smell of death and decay brings an adrenaline rush to the old exorcist. Bernard holds his sun amulet above his head, illuminating the room. 

With righteous fury, he yells out, "By the light of Lathander, I will burn away this darkness!"

Turn 1: Channel Divinity Radiance of the Dawn. All hostile creatures within 30 ft of Bernard must make a DC 15 Constitution Saving Throw. On a failure, they will take 18 radiant damage, half on success. 

Turn 2: He'll cast Sacred Flame on the most injured ghoul. DEX Save 15 or 5 radiant damage. 

Reminder that Bernard has Protection from Evil and Good, and the ghouls will attack him with disadvantage. 

Saves:
1 ... con 3 dex 18
2 ... con 12
3 ... con 6
4 ... con 1

The four ghouls finish clawing their way out, they are scortched from the radiant powers tossed at them, though the last manages to evade the sacred flame that would have caused yet more damage.

"We are beautiful." One of them mumbles out. Another says. "We are immortal..." A third says, "Feed us to make us stronger..." The fourth doesn't bother with words, as they move to attack you.

Bite. Melee Weapon Attack: 3 to hit, reach 5 ft., one creature. Hit: 10 piercing damage.
Bite. Melee Weapon Attack: 17 to hit, reach 5 ft., one creature. Hit: 4 piercing damage.
Bite. Melee Weapon Attack: 7 to hit, reach 5 ft., one creature. Hit: 7 piercing damage.
Bite. Melee Weapon Attack: 11 to hit, reach 5 ft., one creature. Hit: 5 piercing damage.

You are up again.

Bernard is bitten as red blood begins to flow down his priestly robes. Surrounded by the flesh-craving undead, he grips his amulet tighter and prays. 

"Holy Lathander, deliver me from the bowels of hell. Grant your faithful servant the protection of your divine angels; give me the power to banish this darkness!"

Spirit Guardians appear around Bernard once more, slashing at the ghouls with holy swords of fire. 

Beginning of ghouls turns, must do a DC 15 WIS Save or take 14 radiant damage, half on a success. 

All four of the ghouls are cut down by radiant blades, their bodies turning to ash from the power of your god. Soon just the stench of scortched flesh is all that is left of them. (all of them had 6 hp left, so even half damage was going to finish them off.)

Moving to the North you come to a small chamber with nothing in it except for a small dust-covered bed. Chains on the floor reveal this to be some sort of prison, but there is nothing left here of value or interest.

Moving back South you come to the intersection from before. To the East stairs descend and to the South the passage opens up into another chamber.

Bernard is tranquil, kneeling on the floor as the angels' blades cut the ghouls into ash. He whispers, "Lathander protects."

Returning to the intersection, he quickly checks to make sure Lancelot is alright tucked away in his bag before investigating the southern chamber. 

This room is festooned with moldy skeletons that hang from rusty shackles against the walls. A wide alcove in the south wall contains a painted wooden statue carved in the likeness of a gaunt, pale-faced man wearing a voluminous black cloak, his pale left hand resting on the head of a wolf that stands next to him. In his right hand, he holds a smoky-gray crystal orb.

You can see five ashen shadows burned into the walls, with soot marks stretching across the floor toward the Darklord’s statue
A tunnel runs East out of this chamber.

With your observant nature, you see a concealed door in the West wall of this chamber.

Bernard whispers a prayer for the poor souls tortured by the cult. It has been so long since he has seen such cruelty...a lifetime ago. 

He opens the secret door in the west wall, spirit guardians still surrounding him.

It’s basically an ordinary (albeit rotted) wooden door hidden under a layer of clay. The door pulls open to reveal a stone staircase that climbs 10 feet to a landing.

The staircase ends at a landing with a 6-foot-high ceiling of close-fitting planks with a wooden trapdoor set into it. The trapdoor is bolted shut from this side and can be pushed open to reveal the den on the main floor of the house.

Bernard notes the secret staircase to the first floor den, perhaps useful if a quick escape was necessary. 

Then, he continues down the east tunnel out of the chamber.

The eastern passage leads to another chamber...
A chandelier is suspended above a table in the middle of the room. Two high-backed chairs flank the table, which has an empty clay jug and two clay flagons atop it. Iron candlesticks stand in two corners, their candles long since melted away.

Nothing in the chamber itself appears to be worth anything, or of much interest. Of interest though is a door in the East wall and another in the South wall.

Bernard continues through the East door, unfettered. 

Continuing through the Eastern door you find a bedchamber...

This room contains a large wood-framed bed with a rotted feather mattress, a wardrobe containing several old robes, a pair of iron candlesticks, and an open crate containing thirty torches and a leather sack with fifteen candles inside it. At the foot of the bed is an unlocked wooden footlocker.

On the wall of this chamber is two portraits. One shows Mrs. Durst and the other Mr. Durst.

Bernard stows the candles in his pack, but not the torches. He quickly checks out the footlocker before heading to the other door in the south wall. 

In the footlocker you find a folded cloak that feels magical, a small wooden coffer (unlocked) containing four potions of some red liquid, a chain shirt, a mess kit, a flask of alchemist’s fire, a bullseye lantern, a set of thieves’ tools, and a spellbook with a yellow leather cover.

Bernard is impressed at the treasures found inside the footlocker. He has no qualms in just taking all of it given how brutal the cult was. Then, he'll head to the door in the south wall. 

When you go to lift the folded cloak from the chest...

Mrs. Durst, a ghast, bursts from behind her portrait and attacks.

Unlike the ghouls, Mrs. Durst retains the ability of speech. She has retained her memory, but has completely succumbed to her own dark whims. She wears a tattered, once-beautiful red dress, and she wears gold earrings and a golden necklace around her neck. Her lips and gums have gone black with rot, and her smile shines with madness. At this point, she bears only a vague resemblance to her own portrait.

"You dare to steal from me?" Mrs. Durst growls in a hissing, gurgling voice.

She gets a surprise round, but let's do initiative...

Mrs. Durst ... 12
you ... 8

Stench. Any creature that starts its turn within 5 feet of the ghast must succeed on a DC 10 Constitution saving throw or be poisoned until the start of its next turn. On a successful saving throw, the creature is immune to the ghast's Stench for 24 hours. b

The ghast attacks...

Surprise action... Claws. Melee Weapon Attack: 7 (21 if disadvantage on the attack.) to hit, reach 5 ft., one target. Hit: 12 slashing damage. If the target is a creature other than an undead, it must succeed on a DC 10 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Normal action ... Claws. Melee Weapon Attack: 8 (16 if disadvantage on the attack.) to hit, reach 5 ft., one target. Hit: 11 slashing damage. If the target is a creature other than an undead, it must succeed on a DC 10 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

I could not recall if you had protection from evil and good up still, so rolled twice for the two attacks. If you do have it up then take the lowest, if you do not, then take the first.

You are now up.

(I don’t have it, but I have spirit guardians instead up. She takes at least 3d8 save half, maybe 6d8 save half if Bernard can keep up concentration.)

Stench: 4

Paralyze: 6

Concentration: 6

The ghast is able to surprise Bernard and dispel his concentration on Spirit Guardians, but not before being sliced at by the angels. Her first strike hits but fails to paralyze, yet her second strike misses. 

Spirit Guardians: 14 radiant damage, WIS 15 for half. 

Enraged, he yells back at Mrs. Durst. "Your husband is right, you have indeed become a monster! In spawning this vile cult, you have caused the demise of the rest of your family. I will bring you back into the light; I will show you the wrath of holy fire." 

He casts a spiritual weapon and follows it up with a sacred flame. 

Spiritual Weapon: Attack: 17 Damage: 10

Sacred Flame DEX 15 or 12 radiant