Nexialist

"The Key-bearer" as the explorers sometimes call him, dressed in leather either as a humanoid but flesh-less animal or as a fully cloaked figure, followed certain users around silently but occasionally chose to speak, either unintelligible lines that resembled suffocation, or hints about keys and places. Sometimes, he would let others follow him into eerie environments and parallel existences. 

Sometimes he (or it?) would unleash monsters to eat the users and trap them in caves or attics especially in the Animal House world where venturing on the road over the fence turns out to be the worst idea. 

"You have to go INSIDE"  stands out as the most often used line and people have tried since forever to figure out what it means, but to no avail.

"Spread the word" is another, which people have deciphered as being an invitation to bring others into the demon's lair.  

Will you accept the invitation?