The guiding bolt smashes through the shield spell.

The skull drops to the ground with a final scream.

Round 4 : 5 vs 9

 The zombies continue their attack.

Kara Attack: 23 Damage: 6 and 11 Damage: 6 

Griel 16 Damage: 4 

 Stealth 18

 Attack: 26 Damage: 14

 Attack: 14 Damage: 12

 Attack: 26 Damage: 18

 Fire bolt Attack: 22 Damage: 5

 Sacred flame 5

Round 5 : 15 vs 14

 Attack: 27 Damage: 19

 Attack: 21 Damage: 10

 Attack: 15 Damage: 12

 Arcana 17

"Do you have any holy water Vishka?"

 "No. Why?"

 "Because we have just under one hour before this thing regenerates."

 "Are you serious?"

 "Very."

 "That is not good news. I need to rest."

 "I have an idea."

The group scoop up the skull in a bed roll and ties it up with rope. Then they take it back down the passage to the store room they found earlier. 

 Griell has a lock that she places on the door.

The group then head back to the larger chamber where they take a short rest before moving on.

 Blink collects whatever crossbow bolts she can find.

She gives a potion of healing to Griell.

After around an hour a large explosion is heard from the southern passage and a green glow can be seen in that direction growing brighter.

"Time to get out of here!"

 Perception 11

The group hurry through the low tunnel to the east. They have to crouch low to pass through. They emerge onto a narrow ledge  overlooking a large cavern that houses a surging, seething body of water. The rhythmic booming heard throughout the mines is louder here. At regular intervals, a fresh surge of water funnels into this chamber and slams against the wall just below the ledge. The echo suggests that this cave might be one arm of a much larger cavern to the northeast.

Explore 17

Perception 15

"Well this is the source of the sound." Melandra has to shout over the noise of the water. "It appears we need to keep looking."