"Maybe." Elero scans the facade. "Is there any other way in?  There are downsides to going in through the front door."

Mark will pull up his weapon and say, "Be prepared for a fight." 

Before we open the door Mark looks for traps around the door. 

Investigation: 13 + 2 = 15

(Erix has a Passive Perception of 19 btw^^)

When Mark is done with his investigation Erix will open the door

You don't find any traps. Because of the collapsed nature of this passage, the only way in or out of this shrine is through this door.

((Hmm I edited my post, but cant see the edit regarding opening the door^^))

Erix will open the door as no traps are found.

Erx opens the doors to the first room of the shrine, a large dilapidated vestibule. Its walls are covered with chipped, stained friezes portraying some strange events. A heavy curtain hangs across a doorway in the eastern wall, and a door cracked open a foot or so leads to the south.

Do we recognize the art or its style?

Mark DC 20 Religion Check: 6

Mark doesn't notice anything significant initially about the friezes. They seem like generic religious stuff, not really his area of expertise. 

"This seems like some religious drawing, but I know very little of the Gods. If either of you know anything, it may help."

(With so few of you in the party now, I'd let people re-roll checks ability checks even if they aren't proficient. So if someone wants to inspect the friezes even closer go ahead and roll a religion check.)

"Elero, what about you? You recognize these?" Erix asks Elero, knowing his knowledge of this subject is severily limited.

"I'm not overly religious, but..." Elero studies the strange artworks.

Religion check: 9

As Elero steps up to the friezes to understand their import, a glyph lights up and triggers, casting it's negative effects onto Elero. He makes a DC 14 WIS Save: 10 or have Disadvantage on INT Ability Checks and Saving Throws until cured.

At that same time, you hear a noise scrambling out of the room to the south. An exposed brain on four legs comes out of the room and immediately attacks the party, an Intellect Devourer. Initiative:

Group 1: Elero and Mark

Group 2: Intellect Devourer

Group 3: Erix

Elero and Mark are up. The Intellect Devourer is coming for Elero as he was the one that triggered the trap.

And Elero has Disadvantage on INT Ability Checks and Saves.

Elero grimaces, feeling the sting in his brain. "Lesson learned: art sucks.  Stay away from it." He brandishes his daggers and attacks the creepy brain thing.

Magic dagger: 15 to hit and 5 damage

Normal dagger: 14 to hit and 1 damage

The Protector Cannon gives 6 temp HP to any who need them.

Mark will use a bonus action to disengage and move away then he will shoot his crossbow. 

Attack: 14

Damage with sneak attack: 11 + 6 = 17

Both attacks hit the Intellect Devourer but it fights on, looking very beat. It attacks Elero with it's Claws, then tries to invade his mind telepathically. 

Claw against Elero Attack: 23 Damage: 8

Devour Intellect: Elero makes a DC 12 INT Save: 14 or takes 7 Psychic and is Stunned.

You all are up. The Intellect Devourer looks to be setting up to weaken Elero and invade his mind. 

Looks like Elero's set up to have his brain devoured, because he failed his save.  Although I think you have to roll higher than his Intelligence (17) on 3d6 to stun him, but I could be wrong about that.

(I'm using the new stat block, which is just a save. Although you passed it even with Disadvantage.)