Short of entering the chamber to take a closer look (investigation), you all look around from outside (perception)...

• Beegred: 15
• Grithik: 10
• Orubis: 9
• Rodolfo: 5
• Skameros: 18

No one senses any immediate threat from beast or plant.  Beegred and Skameros are particularly curious about the shards littering the summoning circle, in a 'there's something not right with those' way.

Rodolfo will (still from the door) alternate between gust and prestidigitation to remote-clean some of the floor and stuff. Mostly Gust though as Prestidititaion has a lesser range.

Rodolfo remote cleans the summoning circle, gusting the stone shards to the sides and back of the cavern, leaving a 'clear' strip free of debris.

With those shards out of the way of the summoning circle, Beegred will try to spend more time looking at the shards (from a safe distance).
Do they look like shattered remains from a statue, like fingers or arms? (destroyed earth elementals)
Or perhaps they have writing on the surface? (broken arcane seals)

Rodolfo says, "Beegred, you look like you really don't want to get in there. I understand. But we'll get you out again if something weird happens."

Beegred enters the cavern with the summoning circle, weary and watchful, to examine the stone shards...

DC 17 Intelligence (Investigation) check:  21

Beegred can't make heads or tails of the stone shards as his attention is drawn to the power of this place.  The power feels thick to his senses and it vibrates in his innards.

(Does Beegred stay or leave?)

Beegred will step back out, so he can explain to the others what he just experienced.
(With the Observant feat, his description of the sensations are accurate. It is his interpretations that leave something to be desired.)
"... To sum up, it is like a sauna in there, but without the heat."

On a hunch, he asks Skameros to find a large-ish rock, and to roll or toss it into that cavern, without Skameros entering the cavern.
(Not something that would cause a pulled muscle to lift, but a lot bigger than the shards.
Wondering if the "power" is pulverizing big rocks into shards.)

Skameros looks around for an appropriately sized rock based on Beegreds recommendations and tosses it in to the cavern

An interesting experiment, but nothing happens to the larger rock that Skameros throws into the cavern.

"... Any moment now... any moment..." Beegred continues staring at the rock that Skameros threw.

After it is clear nothing is happening, Beegred will dash into the cavern, try to grab one of those suspicious shards, and run back out before inspecting it closely.

Orubis steps closer to the circle…

Beegred dashes into the chamber to snatch up a shard and as he hurries to exit, he runs into Orubis who enters.

DC 17 Perception check (disadvantage for those not in the chamber):

• Beegred: 21
• Orubis: 22
• Rodolfo: 4
• Grithik: 11
• Skameros: 10

The last few minutes of commotion has caught the attention of a pair of beasts, who emerge from small tunnels dug throughout this larger cavern.

(I will set up initiative and a map ASAP)

Rodolfo goes first, but can not see the creatures.  

Rodolfo perception check (sounds) vs. creature stealth:  6 vs. 2

Rodolfo is unaware of the creatures, intently watching Beegred and Orubis.

Orubis's turn.

Orubis does see the creatures emerge from nearby small tunnels -  a pair of multilegged reptilian horrors with eyes that reflect a eerie light in the darkness.

(Orubis has the option to avert his gaze and have the blinded condition for any actions towards these creatures, OR make a DC 12 Constitution saving throw before taking any further action.  On a fail Orubis will be restrained).

Orubis will avert his gaze, if he recognizes the things, and cast spike growth with the near edge at M, and filling all around the Basilisks and heading south to cover most of the room (20 feet radius is huge)

Beegred averts his gaze, careful to only use his peripheral vision to watch for the approach of the reptiles.
To Rodolfo and the others: "Two basilisks have emerged. Be careful where you look, and let Orubis's spell do its work."
Beegred will hold his current position. He readies an Attack action, if either of the basilisks moves within melee distance. (Attacks will be with disadvantage.)

(Looking at the initiative order, next is Skameros, who is tied with the reptiles.)