Randall:

Looking to the bas-reliefs and the holes therein, Randall considers how these people met their fate...Perhaps a monster, or perhaps some foul gas from these zodiac killers caused confusion and combat...He moves over to the closest one and examines the hole, saying, "When I see holes like this in walls, I can't help but wonder what might come out of them..."

Perception:  12

Unless he sees something concerning, Randall will get a candle out of his bag, break off a piece of it, and wedge it into the hole to make an airtight seal with the wax.

@Randall

You notice just before you plug the hole that is quite damp and saturated with water.

Randall:

"Look at this," says Randall, as he sees the wetness of the hole, though it is only water.  "Here, use wax from these candles and plug the holes...then we can jam them shut with the hafts of some of these weapons lying about.  If this place intends to drown us, let's make it hard for it to do so," he adds, suiting action to words and doing his best to plug the hole at which he is currently standing.

Randall manages to plug 1 of the 12 zodiacal holes, there are 11 left.

Water begins pouring into the chamber from the 11 zodiacal holes.

It will take at least 1 person 1 round to plug 1 zodiacal hole.

Roll for Initiative: 12

Also roll a Perception check vs.: 13

Tempe looks at the holes calmly "What will so much water do? Do you think this is what killed them?" he looks and thinks how much time they have before the chamber is full. 

Randall’s Initiative:  19

Randall has 9 remaining candles, easily broken into pieces…come grab one!

Initiative 15

Randall’s Perception:  20

Perception 20 in gl

Pepin initiative: 6

Perception: 10

Quote from Brian_Avery >>

Randall manages to plug 1 of the 12 zodiacal holes, there are 11 left.

Water begins pouring into the chamber from the 11 zodiacal holes.

It will take at least 1 person 1 round to plug 1 zodiacal hole.

Roll for Initiative: 12

Also roll a Perception check vs.: 13

Initiative: 18

Perception: 10

Z follows Randall's example and begins plugging the holes.

He asks Irraxan to do the same (Irraxan can take his own actions on his own turn)

There is already an inch or two of water and enough of the group has beaten the DC so the the whole group notices a sloshing in the water, someone or something is invisible and nearby.

You've managed to ruin it's attempt to hide and ambush so there will be no surprise round and we'll proceed with regular initiative.

Quote from Brian_Avery >>

You've managed to ruin it's attempt to hide and ambush so there will be no surprise round and we'll proceed with regular initiative.

(It couldn't surprise us anyway, could it? Given Z has the Longsword of Warning (post 1714). Yes, he doesn't actually use it in combat - preferring spells - but its magic properties still apply:

Longsword of Warning

This magic weapon warns you of danger. While the weapon is on your person, you have advantage on initiative rolls. In addition, you and any of your companions within 30 feet of you can’t be surprised, except when incapacitated by something other than nonmagical sleep. The weapon magically awakens you and your companions within range if any of you are sleeping naturally when combat begins.

Or am I missing something?)

True. I had almost forgotten. So you guys actually had double protection there and at least one of them nullified it.

There is only one creature... or thing, that is invisible within the chamber with you.

Petcan initiative 5 

Round 1

Initiative Order:

Zenodor
Tempe
Petcan
Invisible Thing?
Randall
Pepin

Z doesn't really have anything to deal with invisibility. He follows Randall's example and spends his first round plugging the holes. He asks Irraxan to do the same (Irraxan can take his own actions on his own turn).

(How fast is the water rising? We want to be able to stop it before it causes us real problem or drowns us. But a little more water might be useful to see whatever invisible creature is hiding in here. so we probably want to plug most of the holes and leave a few for later.)

If the thing attacks him, Z will use the skill: 

Entropic Ward :Once per short rest, when a creature makes an attack roll against you, you can use your reaction to impose disadvantage on that roll. If it misses you, your next attack roll against the creature has advantage if you make it before the end of your next turn.

Tempe summons 8 giant(medium sized) wolf spiders above the invisible one.

They all walk on the ceiling and rain down on the invisible creature

1Attack: 6 Damage: 3 DC 11 Constitution saving throw poison damage 5

2Attack: 14 Damage: 6 DC 11 Constitution saving throw poison damage 7

3Attack: 12 Damage: 5 DC 11 Constitution saving throw poison damage 10

4 Attack: 8 Damage: 2 DC 11 Constitution saving throw poison damage 5

5 Attack: 11 Damage: 6 DC 11 Constitution saving throw poison damage 2

6 Attack: 19 Damage: 3 DC 11 Constitution saving throw poison damage 8

7 Attack: 20 Damage: 3 DC 11 Constitution saving throw poison damage 11

8 Attack: 5 Damage: 5 DC 11 Constitution saving throw poison damage 3

"Even if my friends are ineffective, aim near the spiders they have blindsight."

He tells pinky to stick a candle in the holes