Ash looks around. Something triggered the spell or someone is here still. 

I give you power and you stay here jabbering like a dumb kobold. Wasted power. Then take it back.

arcana 10

"What happen to Og's axe? Why Og ears ringing? Where owlbear go?" Looking around the room for tracks or another door/entry way.

Perception 12

(For Ash's Arcana:)

While there's no loot on an owlbear, paradoxically there's more loot on this owlbear illusion — the rock enchanted with Silent Image is only about the size of an orange. The enchantment seems to be permanent.

(For Og's Perception:)

The tunnel continues for another 80 feet of curved passageways, until it ends in a large door, filling the entire passageway. Something's carved into the heavy wood in that same fancy goblin script: "Welcome to Neeblenoc, the highest goblin city in the world. Go away."

There's a heavy iron knock ring and three interview slots — one at a goblin's height, one at a human's height, and one at an orc's height.

Beside the ring is a plaque with instructions: "Knock once for each foot of your height."

yelling "I FOUND A DOOR OVER HERE." Not being very good with reading or counting Og will knock 5 times.

There's muffled argument from the other side of the door, barely audible though whoever it is must be shouting.

The human height slot opens, but it's not an interview window at all, it's a dart trap.

(Og needs to make a Dexterity saving throw, but with advantage because of his Danger Sense.)

dex save 12

The dart nicks Og (2 piercing damage). There's more muffled yelling on the other side of the door.

(Any chance Yahg, as a Goblin, would have any knowledge of a set up like this, and if there is any kind of 'secret knock' or other code to get through safely?)

(If I have to roll my Intelligence is only 8 xD)

Yahg puzzles over the door. The fancy goblin language is somewhat strange to him, but the word "welcome" uses familiar speech — grammar patterns that one goblin would use toward one of equal status, and "go away" uses fearful speech, which goblins use when speaking to goblins of a higher caste or bigger species.

After seeing Og's luck, Darksniffer draws his bow and an arrow, walks to the door and knocks three times. After knocking, he draws his bow and steps to the side, aiming the arrow through the slot for whoever pokes their head through.

"I have to say this cave has not been very nice so far." States Og as he leans against the wall wiping the blood off his arm.

The lowest slot opens, and a goblin with excessive ear hair, wearing some sort of uniform, challenges the party.

"Who's there? Show yourself!"

Darksniffer shoves the arrow into the goblin's face, "Me Darksniffer, kill many Bigs. You open, let us in."

The gatekeeper's eyes get wide with terror. "Don't hurt Karbo! You a Gorog shooter?"

From the corner of the gate Darksniffer can't see, another voice cackles in Goblin. "Shoot Karbo! Shoot Karbo's dumb face!"

Ash casts comprehend languages

As soon as the incantation is complete, Ash understands the goblin argument. If he touches the door, he also understands the inscription.

"We no with Gorog, we run from him. See Death-Pit in road and trail to here. You no hide tracks good, need more like thick bushes. Let us in."

Karbo is an adult goblin, but begins to cry loudly at being held at arrowpoint. The other goblin inside the door, wearing the same uniform, is doubled over with laughter. "Shut up, Grippa!' Karbo wails. "Don't kill Karbo! Put your bow down! Karbo can let you in!"

Darksniffer lowers his bow once he sees the door open and no nastiness comes out.