First time DM 

I am running a campaign and one of my rogues just got offered to join Asmodeous' Cult by making a deal with the devil himself. It makes sense to me that if he decides to join he is basically getting a patron. Can I tell him he has to multiclass to warlock once he joins? And is it messed up if its a surprise after he takes the deal? Or should I mention it before and come up with a different solution if he is not into it? 

Well, you can, the DM is free to make whatever rules are needed for the game, but whether you should is a separate question. I would probably avoid doing so (and I would certainly not ambush the player with the information) because bog-standard cultists don't have any apparent class and Asmodeus probably has uses for servants who aren't clerics, paladins, or warlocks (though if the character does multiclass into a class where it's applicable, fair to require specific subtypes).

You ask the player if they want to have Asmo as a patron with all the bells and whistles. Let them know that this will involve dipping a level into Warlock. It work like regular and they would take a level in Warlock at the appropriate time.

If they don't want to, they can still honor Asmo just with no side benefits.

Remember Jurassic Park?

”Your scientists were so preoccupied with whether or not they could, they never stopped to think if they should.”

— Ian Malcolm

Now, making membership conditional upon a choice to become a Warlock isn’t necessarily forcing them to do it. It’s their choice, they could walk away. But another thing you could do is after they join, you then give them permission to Multiclass. Remember, Multiclassing is an optional rule. You can restrict it to only those characters who do something specific to justify it, like swearing allegiance to Asmodeous.

Quote from Evil_Lynn >>

First time DM 

I am running a campaign and one of my rogues just got offered to join Asmodeous' Cult by making a deal with the devil himself. It makes sense to me that if he decides to join he is basically getting a patron. Can I tell him he has to multiclass to warlock once he joins? And is it messed up if its a surprise after he takes the deal? Or should I mention it before and come up with a different solution if he is not into it? 

 
If the scenario was nothing about multiclassing or warlockery was hinted at, I took deal and DM said "now you have to start multiclassing into warlock", and I did not want to be at all, then I would be extremely pissed. The DM sneak-attacked all my character ideas and ambitions, and took part of my character away from me without reason. If that DM refused to understand, I would leave the group.

If the DM told me, be it through in-game RP or as DM to player (which is preferable - best to be clear), that if I took the deal I would have to multiclass into Warlock, I could then choose to agree and take deal or not agree and not take the deal. That would be perfectly fine.

What I would do as a DM is tell the player if they take the deal they will get the Magic Initiate (Warlock) feat for free and will have the choice to rely more on the gifts of Asmodeus, taking Warlock levels, if they want, but I choose the subclass for them. I would let them know this before they agree. They could take the deal, get the feat and continue levelling in their current class if they want, or they can take levels on the warlock class. The deal is better as the RP way of allowing the multiclass, not forcing/tricking them into it.

I am not a fan of a DM taking away all player agency over their own character. There are many cultists who are not warlocks, so forcing warlock on somebody just strikes me as a dick move with no lore-justification. Always better to say "you may" rather than "you must".

Just for clarification. I know i can from a rules perspective. But I when I said "can I", i meant should I. Im just trying to get feel for what is acceptable and what isnt from a players perspective. We are actually all new. So I never played, though I am DMing. 

I actually already told the player to look at the warlock and see if he would be interested. So I am not surprising anyone. But wanted to get some opinions of more experienced ppl. 

I'm not a fan of forcing a Player to multi-class their Character.

However, making the Player the offer is a different story. If it were my game, I'd tell them that if they join the cult, then they get such-and-such perks ( allies, information, resources - so long as they are on good terms with the cult ), and they invoke such-and-such responsibilities ( the cult gets to call upon them for information and services, or they lose their good standing with the cult ).

That's what they get with their current Character build.

However, if they want to multi-class into Warlock, they get that, plus all these other class abilities and perks.

Then leave the choice up to them.

Oh, the feat thing is a good idea.

I have no problem with you saying, "Joining this cult requires you to take a level of Warlock," that is a condition of joining the cult. Then the player can decide if he wants to join and take the level. Asmodeus is not the friendly sort so this won't be the last of the demands made of this character.

However, I would not be a fan of, after the player joined and with no warning, saying "Next level you have to take warlock." That would not be cool.

Something like the Magic Initiate Feat is a nice little way to represent their connection with Asmodeus, but isn't too broken. A decent rare magic wand will give a player more versatility than the feat, although the benefit of the feat is the customizability. Maybe it will also motivate them to dip into Warlock... Eldritch Blast is the obvious motivator. It's a good cantrip, but to make it good enough to compete with just, y'know... buying a bow, you need Eldritch Invocations to beef it up.

Anyway, I think Warlock is one of the classes that's easiest to justify a character suddenly multiclassing into. It's hard to say, "Oh, my Rogue has actually been studying magic this whole time in the background, so now I'm part wizard"'. Warlock just needs one interesting meeting with a powerful magical being to handwave all your magical powers that arrive after that point. Plus it's easier to multiclass into if you're already a spellcasting class, since Warlock spell slots are easier to manage in addition to a second set of dedicated spellslots.

Warlock doesn’t have to be the only option. They could swear an Oath to Asmodeous an become an evil Paladin, or Worship him and become a Cleric. Why not?

Not sure what divine domains match Asmodeus; Conquest is the obvious paladin choice.

just be clear on what joining the cult entails. be mindful of the other players in the group as well. sometimes these things change the party for the worse.

also, there are factions like the Harpers etc...who have ranks of membership. you could start him at rank one....very small perks...perhaps gets a connection with the cult etc. and then have him work his way up, perhaps only when you get to rank three does he have to take a level of warlock. 

also have repercussions if he betrays the cult and has a change of heart....idk if his level of warlock would work if that happens.

Does it have to be a warlock?  I mean, could Asmodeus grant Rogueish skills or something instead? I think he’s all about trickery and mayhem, no?  But I do see the point about being able to easily keep track with warlock spells. 

Ask him and if he says no then don’t make him multiclass.

Maybe instead give him some form of fiendish boon that boosts his power (in a way that the player would like that is not OP that fits the character.) by giving him warlock like abilities, however at the consequence of having to follow asmodeus. If he does not follow the commands of asmodeus then devils could be after the party trying to kill the rogue and those defending them for taking their overlords power. The fiends may stop attacking if the rogue somehow gives back the power of the boon.

Not everyone who makes a deal with a devil becomes a warlock. Warlocks are people who specifically make a deal for warlock power, but some people make deals for material wealth, the heart of their crush, social/political clout, luck, etc.

Even if the rogue made a deal on the condition of "help me get stronger", you can always have the devil give them *an* singular magic ability they can use once per short or long rest,  or a magic item that increases their power. Or you could take an Eldritch Invocation from the warlock class and grant it to your rogue. Basically there's a lot you can do before getting to "yer a warlock Harry" which the player may or may not be thrilled about. 

Also, the carrot works better than a stick.

I would offer them something extra - for example an extra cantrip - as an inducement to convince them to take it.

Don’t make them multiclass. Not everyone who makes an Infernal Pact is a warlock. Being a warlock just means you get arcane spells and other class features in exchange for making the pact. The character can stay as just a rogue and gain some other mechanical benefit for joining the cult.

Id be extremely cautious taking away my player's ability to make choices.

The way I did it was I gave them a level in the class but I didn’t count against Xp they need to level up or capstone for the class they were. The player would end on level 21