Vows for monks

Guys I’m a relatively new dm and I want to make one of my players (who is a monk) make a sort of sacrificial vow to be able to enter a room. Do you have any ideas?

Why do you want the monk to make a sacrificial vow?

Are you making others do a vow?  what is it about the monk to do this?   What is the monk does not do a vow, are they kicked out of the game?  Do others enter a room with no vow? With a bit more detail it might be easier to determine what you are looking for.

Sorry about that! To clarify, it’s this room that was established as a sort of worship room, later turned into a hideout. I have decided that they will all need to make a vow to enter the room, probably based on some key ideas. If they don’t enter, then they don’t get to know what’s inside, but I know they will want to enter lol

Asceticism, giving up their valuable possessions, cutting emotional ties. 

Well, ask your player what's most valuable to their character, and find someway to incorporate that.

Ok, thanks!

There are a lot of rules in D&D. And then there's something else that we call "flavor". Flavor is just a design choice that doesn't effect the game mechanics at all. For example, we all have an idea of what leather armor looks like. But if a player says they want their rogue's leather armor to look like cowboy chaps with tassels and a long heavy duster coat, that's flavor. It doesn't effect the armor class, or the function, or the dice rolls, or anything. It's just a design choice. Whether a player wants their monk to dress like a Shaolin Buddhist or like John Wick is just flavor.  Vows are flavor. Sure, in extreme cases the DM can use a violation of those vows to enact some form of punishment upon a character, but it's mostly just flavor. 

So before we decide what the specific sacrificial action will be, it may help for you to think about the monk character and where they trained and what kinds of rules their trainers or their temple or their god imposed on them. Some vows are big life decisions, like poverty or celibacy or silence. But that's a bit much for just entering a room. Maybe there's a small statuette of the monk's god in that room, and the monk has to lay a piece of food in front of that statue every time they enter the room. Or maybe the monk has to stand in the doorway and recite a certain poem or mantra for a minute or so before they can enter. Or maybe no one's allowed to wear shoes inside the room. Or no one is allowed to wear any red clothing in the room. Maybe there's a small bowl of water just inside the doorway, and anyone who enters must first wash their face in the bowl. And maybe that water has to be rainwater, so whenever it rains, the monk has to stand outside for an hour filling the bowl in the rain. Or maybe the monk has to burn a certain kind of incense in the room, and that incense is rare and expensive and keeping the room supplied eats up a good portion of the monk's treasure. If the monk follows a very strict discipline, then maybe no one is allowed to laugh in that room, and if they do they must apologize and then leave. Maybe each time the monk enters the room they must cut off a lock of their hair and burn it in front of the statuette of their god. If the monk is dedicated to helping others, then maybe there's a book inside the room and each time the monk enters, they have to write in that book the name of someone they have helped since their last visit to the room. So they can't enter unless they have helped someone recently. 

Just spend some time with the player talking about their character's beliefs, and brainstorming ideas. There are no wrong answers. It's just flavor. This is a fantasy game built on imagination and having fun. Just have fun with it. 

Thank you so much! It feels like such a fine line to walk between D&D mechanics and flavor. This clarified a lot!