PREFACE:  I am just finishing up a level 0 adventure for 8 players, based on 1e (Treasure Hunt).  They are getting ready to level up to level 1 where we will move to 5e rules.  Adventure is based in the Korin Archepelego, and characters are heading towards the island of Caftenor.  This campaign is a "good vs. evil" story based upon the Seven Deadly sins.

QUESTION:  My party has an Aasimar and a Tiefling who I am hoping to drive a wedge between for the first 5 or 6 levels before having them rebond at a later point.  I am considering something like having the Tiefling enter into a demonic contract which places the Aasimar under threat.  Point is, I want to divide these two characters in order to present later scenerios to get them to reform their friendship just before the major battle between angels and demons.  Any thoughts on how to creatively drive this wedge?

Maybe talk this out with your players. If they’re “in on” the direction you see this going, it might be a more fun experience for all of you, not to mention a better story! I bet they’ll have some good ideas too, and they won’t feel railroaded into a dynamic you chose for them. The best creativity comes straight from the group!

Edit: in case this wasn’t clear, I totally agree with the people saying this is a very bad idea unless the players say they want to do it, I just didn’t want to put it as directly!

Quote from brewdude1966 >>

Any thoughts on how to creatively drive this wedge?

 Talk to the players. Find out if they want to do something like that. If they don't, abandon the idea; if they do, let them decide the cause of the conflict. Forcing something like that on players will just annoy them.

Got to agree with the above, you shouldn’t force arcs upon your players because you like the idea, especially one designed to bring tension between two pc’s.

you have agency over and design the environment and the encounters and the story 

the players get the agency over character direction

Quote from brewdude1966 >>

QUESTION:  My party has an Aasimar and a Tiefling who I am hoping to drive a wedge between for the first 5 or 6 levels before having them rebond at a later point.  I am considering something like having the Tiefling enter into a demonic contract which places the Aasimar under threat.  Point is, I want to divide these two characters in order to present later scenarios to get them to reform their friendship just before the major battle between angels and demons.  Any thoughts on how to creatively drive this wedge?

I highly recommend against this. Although talking to your players is a minimum, unless you have players who are really into this idea AND who are experienced and skilled enough as RP gamers not to have this utterly destroy the party, you are much more likely to wreck the entire campaign than to have a good result.

This sounds to me like you have the plot worked out already and you just want the players to play their parts, like a play. This is a roleplaying game, not a script you have written that the players are acting out. You simply cannot manipulate events with a plot like this and expect it to work when other human beings who have their own ideas about their own characters are involved. Players individually will hardly ever do what you expect as a DM and when you put more than one of them together, as a group they quiet literally never will do what you expect. You can't control the snowball once it starts rolling downhill, and the players could very easily (I would argue, more likely than not) RP themselves into a corner out of which there is no possible chance they will get together.

I'd say what you have sounds like a great story for a novel. I would not try to get this to happen in an RPG with other players controlling the two principal characters. Even if they initially agree to doing what you want, the RP is not going to happen just between them -- there are other players at the table. And again, they might RP off into directions that prevent them from ever coming back together. In the meantime you have have the rest of the party taking sides, and are you sure not just these two principals but EVERYONE at the table is this mature and experienced and most significantly skilled at RP not to have it cause a problem.

You are flirting with catastrophe here. It requires a tremendous amount of very skilled RP not to have this turn into a major issue (or worse utterly wreck the campaign) -- I have known very few players who have the skill to pull something like this off, and I'm not sure I would ask it even of the one or two people that I have in mind as possibly capable of it. It's asking for trouble, IMO.

TL;DR -- the campaign really shouldn't be the DM's story -- it should be the PCs' story. This sounds like YOUR story that you are going to try and get THEM to play out, rather than letting them play out their own story. And on top of that the story is likely to cause trouble rather than a fun time. I urge you to reconsider.

Animosity between player characters is a headache. My group has a lawful good bard who is a folk hero and beloved by the sword coast... and a sneaky former sailor rogue who has been known to use mage hand to steal the last coppers from old ladies on their way to buy bread. Those two are constantly  at odds with each other,  snide remarks, practical jokes, and tons of petty bickering, to the point where I as the DM have to interrupt and say “Annnnyway, moving on...”

it was funny at first, but now it’s just annoying. It’s hard to see them as an adventuring party when they fight more with each other than they do monsters. I wish now I had nipped it in the bud at the beginning.

I gotta chime in on this one, talk to the players, but deliberately manipulating them against each other is a terrible idea. That doesn’t lead to a fun contrast of stories but to two people staring at each other like strange cats over the table. That level of awkwardness does not lead to good role playing.

Just to re-emphasize, to me one of the red flags is the DM saying "I want the story to start this way, proceed this way, and end this way." 

I feel like Ian Malcolm in the original Jurassic Park movie: "The level of control you're trying to achieve here is... not possible."

It is entirely unrealistic to expect players to play out hating each other, then at a predetermined time come together and become buddies. Doing it without telling them is an extremely bad idea. Even having them in on it is a bad idea. 

In the first, the players may not choose to hate each other because of demon worship or might choose not to worship the demons. You can't make them do it.

In the second, real person bitterness can make its way into the game even if they are faking it. You lose one of the greatest things about the game if you force roles on you players. Real play, genuine reactions - these are the things that make RPGs so fun. Following a script is not fun. 

Again, considering your plan to have the PC hate/dislike each other, it can be harmful to real world friendships.

To repeat what everyone else has said, this is a discussion to be had with your players.

A campaign I currently play in does have a similar type conflict between myself and another player. During a zero-session the DM discussed with us separately matters of our backstory to sow the seeds of what he hoped would be conflict but he did not make us RP a certain way or let us know about the other player's involvement. My group is also excellent at RP so when they events unfolded, it was fun to RP and over the course of a few months of sessions we solved our problems . The PC were friends before so now this has really solidified their friendship. But when we started this campaign, the DM let us know that there would be ~drama~ between players and its our responsibly as players to work cooperatively and create a happy table environment (i.e not confusing PC drama with the actually players). 

But how to create this wedge? In our campaign is was a breaking of trust. One player had create an entire fake persona (typical changling, go figure) and when that came to light the other PC felt lied to & the trust was broken. They slowly rebuilt their relationship. 

Or rather than a pact that directly in dangers the Aasimar, perhaps the pact dangers something the Aasimar cares about (family, homeland, beliefs). Or to switch it up, the Aasmiar has been given a task by their god to do something that the Tiefling disagrees with. 

In the second, real person bitterness can make its way into the game even if they are faking it.

We know that this can be true even of professional actors who have been trained not to let on-screen shenanigans move into the world of off-screen. How many actors and actresses have played love scenes on screen and then 6 months later you find out they got married or are dating? Even professionals cannot just "turn it on and off" completely.

This is why I say that you need people who are extremely skilled at RP to avoid a problem and maybe won't even be able to avoid it then.

I'd throw in another vote for don't do it. Don't try to dictate player actions. I mean in the original, what if the tiefling doesn't want to enter into the pact in the first place.

The closest thing you might be able to do, would be to have an NPC, who you control do something similar. Like have them start as an enemy and become an ally, or vice versa. That can actually be pretty cool, and you still allow player agency, because the players will decide how close that want to allow this NPC to get to them, if at all.

I agree with Xalthu. If you've really invested a lot into this plot, it might make sense to do something similar with an NPC. But as others have said, don't force it to end the way you want. Embrace the fun of telling a story together and just see where it goes.

Quote from Webzpinner >>

Animosity between player characters is a headache. My group has a lawful good bard who is a folk hero and beloved by the sword coast... and a sneaky former sailor rogue who has been known to use mage hand to steal the last coppers from old ladies on their way to buy bread. Those two are constantly  at odds with each other,  snide remarks, practical jokes, and tons of petty bickering, to the point where I as the DM have to interrupt and say “Annnnyway, moving on...”

it was funny at first, but now it’s just annoying. It’s hard to see them as an adventuring party when they fight more with each other than they do monsters. I wish now I had nipped it in the bud at the beginning.

 I have exactly this problem too, and it really does bring the game down sometimes.

The party rogue just plays as a loud mouth and dirtbag. The druid is deeply annoyed by him, all the time, he and the paladin sometimes end up slapping at each other. The fighter just doesn't engage. That would all sound fine, but the three non-rogues all genuinely dislike the character (in game) and it's hard to justify why he's even with the group anymore.

I recently gave him a sub-quest where he was meant to retrieve an amulet from a dungeon for a lich who is trying to compel him (and who the rogue is willingly working for). The druid got hold of it first by chance. The druid won't give it up, and the patron's undead minion is now demanding it back. I'd hoped the rogue might come clean about why it's needed, or maybe they could destroy it or... nope. The rogue wants to steal it. The druid keeps it hidden at all times.

These stories just give more examples of how party division can be the death of the game, rather than a fun in-character experience. Everyone has to be on board from day 1, and sometimes even then, it all will blow up. It's much better not to do this.

I just had a session where one of my PCs tried to kill another. So I have verified success with this. But I didn't really do much. I just encouraged the rogue to steal from the ranger at several opportunities, and he was game to play along.

I'm starting up a new plotline where I have pitted one of my characters against each other, unbeknownst to either. I simply had the questgiver privately give them conflicting instructions, and warn them to keep their secret marching orders secret. What I like about this conflict is that it can be resolved amicably or inimically.

Quote from Webzpinner >>

Animosity between player characters is a headache. My group has a lawful good bard who is a folk hero and beloved by the sword coast... and a sneaky former sailor rogue who has been known to use mage hand to steal the last coppers from old ladies on their way to buy bread. Those two are constantly  at odds with each other,  snide remarks, practical jokes, and tons of petty bickering, to the point where I as the DM have to interrupt and say “Annnnyway, moving on...”

it was funny at first, but now it’s just annoying. It’s hard to see them as an adventuring party when they fight more with each other than they do monsters. I wish now I had nipped it in the bud at the beginning.

 To each his own. That sounds like a fun campaign to me.

Quote from Sanvael >>

Quote from Webzpinner >>

Animosity between player characters is a headache. My group has a lawful good bard who is a folk hero and beloved by the sword coast... and a sneaky former sailor rogue who has been known to use mage hand to steal the last coppers from old ladies on their way to buy bread. Those two are constantly  at odds with each other,  snide remarks, practical jokes, and tons of petty bickering, to the point where I as the DM have to interrupt and say “Annnnyway, moving on...”

it was funny at first, but now it’s just annoying. It’s hard to see them as an adventuring party when they fight more with each other than they do monsters. I wish now I had nipped it in the bud at the beginning.

 I have exactly this problem too, and it really does bring the game down sometimes.

The party rogue just plays as a loud mouth and dirtbag. The druid is deeply annoyed by him, all the time, he and the paladin sometimes end up slapping at each other. The fighter just doesn't engage. That would all sound fine, but the three non-rogues all genuinely dislike the character (in game) and it's hard to justify why he's even with the group anymore.

I recently gave him a sub-quest where he was meant to retrieve an amulet from a dungeon for a lich who is trying to compel him (and who the rogue is willingly working for). The druid got hold of it first by chance. The druid won't give it up, and the patron's undead minion is now demanding it back. I'd hoped the rogue might come clean about why it's needed, or maybe they could destroy it or... nope. The rogue wants to steal it. The druid keeps it hidden at all times.

 You should both talk it out with your players. If the players aren't having fun with the conflicts, or you as the DM aren't having fun, discuss whether you want to collaborate on a way to steer the plot away from this kind of dynamic.

Agreed with the above.

Unless both your players are willing to role-play this out, you're just damning the campaign to end in total failure. 

Our DM wanted to try this scenario between myself and my best friend, and we realized that it would end in disaster. While our characters looked like a "perfect couple" she wanted to find her missing husband and I wanted to be the protector of our realm between an evil god and society.

Meanwhile, in another campaign with different characters in a completely different system and genre, our characters are long time lovers and friends. 

Without player involvement, you're dooming everything. 

my first though is...don't do that.  let your players play their characters.  the biggest thing that DMs fail to do is to understand that your grand story is rarely going to work out like your vision.  Just let the players roll with things and see where the chips fall.