You find some tracks from Harpen leading into an alleyway next to an abandoned apartment complex before entering a door, will you open the door trying to be sneaky or will you open it normally?

sneaky

I'm gonna say that Harpen doesn't notice you because he is very focused on what he's doing, what you see is Harpen kneeling down and drawing a large circle with a large symbol in it that appears to be multiple eyes each of which are tied together with some kind of thread with a piece of chalk. After that Harpen lights the candles that are surrounding the circle and then kneels in front of the middle of it before taking out his small knife, rolling up his sleeve and cutting his arm a little bit before taking the blood from the knife and putting it in the middle of the circle. What do you do?

Daimion will try and find out what Harpen is trying to summon while taking out his shortsword

Make either an Arcana or Religion check please.

17 on religion

Okay so you recognize this as a ritual that is usually preformed by The Organization of Exceptional Mailpeople, which on the outside sounds like a club for mail messengers who are exceptionally good at their job but on the inside is actually a cult which follows an unknown fiend, they must do a blood ritual where they sacrifice some blood for protection twice a month or else be hunted by the fiends shadow servants.

Daimion will walk out of the door and find a dark building to go into

The nearest dark building is in fact right next door, it is a large warehouse.

Daimion will go into it and barricade the door before taking out his old cultist book.

Alright then, you take it out and out the corner of your eye you think you see a humanoid shadow at the back of the warehouse before disappearing.

Severon will say in a language his cult knows, "Well, just like old times." before putting the book on the floor and removing his shirt.

Okay so I'm gonna need you to tell me all the information about this cult and also you notice more shadows out of the corner of your eye appear.

My Cult is a cult that summoned ancient demons, and evil beings for the chaos of the world. The only thing that is different with Daimion is that he was a cultist the actually was able to summon demon lords and Kings instead of regular demons only. He also has been dead for 100 years.

Okay then, I would like you to make a wisdom saving throw for me with advantage.

23

Alright then, you realize at this moment that those cultists were an illusion and that you haven't joined the cult again, basically you don't have to do whatever ritual you where about to do.

Daimion is still going to take out one of his daggers and cut his finger to make a pentagram on the ground.

A window breaks out of nowhere and more shadows have appeared.

Daimion will look at the shadows and take out his candles.