I like brainstorming new builds and a blade warlock sounds fun. Goblins for my race and The Fiend fit what I'm going for mechanically, but I wasn't sure how to play that. The PHB description of fiends make them sound like a BBEG out to destroy the world, which isn't a role I want to play. I'm okay with not playing a good character, but I don't want to play evil either, so here is what I have so far.

Mozz has spent most of his life raiding villages and caravans with his goblin group. There was a power struggle in the group and one of Mozz's rivals attacked and almost killed him. He wanted to use magic to get stronger, so he looked through some of the magic books they had looted and ended up contacting a powerful fiend, but the fiend had bigger plans. It wants to destroy the world, but it's plans aren't ready yet and he doesn’t want some other world destruction plan getting there before he does. So the fiend gave Mozz visions of other destruction plans to stop.

Does that make sense? Does that fiend sound like a demon lord, an archdevil or some other type? Hopefully, that backstory won't conflict with the DM's world in whatever game I use it in.

"If anyone's going to destroy the world, it's going to be me," eh?  Demogorgon's about crazy enough for that; and it actually makes sense coming from one who wants to rule the world, rather than destroy it, such as Lolth. 

Seconded to Matthias' option. A devil who is smart enough might also be able to trick your goblin into believing his is a more benevolent being, and send him on missions or give him goals that Mozz maybe doesn't realize make him a cog in the "world destroyer" machine. This also gives your character some super interesting role play potential and your DM the back pocket tool of an amazing reveal for your party wayyy down the road.

For Fiend Warlocks, I like the idea that a devil tricked your character into some nefarious scheme to eventually harvest the character's soul or trick them into turning them evil. (A deal with a devil, if you will.) I hadn't thought of the idea that the devil would influence the character's quests though; that's a cool idea and really opens up a lot of possibilities.

my thought would be to have your familiar being both your servant AND your keeper. your familar serves you aslong as you dont go against the true master

I'm working on a Warlock Halfling.  For me, his RP is going to be focused on his fall...  

I have it worked out with the DM that the character will start out saccharine sweet.  The more over the top adorable and well mannered I make him at the start, the more dramatic his fall...and the more memorable his existence.  As he levels up, he will start to become a little more evil.  He won't get his imp familiar till level 3, so I have some time to nauseate everyone at the table with kindness.  At Level 3 he's going to start pestering the cleric to consider the benefits of serving the dark one.  The imp may start appearing on people's shoulders to dispense horrible advice when they have to make critical decisions.  At Level 4 he may be open to torturing a captive or two.  

I have time, but I am looking for increasingly evil ideas to illustrate the fall without interfering with game and/or narrative structures if anyone has some tips.

Quote from Zamfir >>

I'm working on a Warlock Halfling.  For me, his RP is going to be focused on his fall...  

I have it worked out with the DM that the character will start out saccharine sweet.  The more over the top adorable and well mannered I make him at the start, the more dramatic his fall...and the more memorable his existence.  As he levels up, he will start to become a little more evil.  He won't get his imp familiar till level 3, so I have some time to nauseate everyone at the table with kindness.  At Level 3 he's going to start pestering the cleric to consider the benefits of serving the dark one.  The imp may start appearing on people's shoulders to dispense horrible advice when they have to make critical decisions.  At Level 4 he may be open to torturing a captive or two.  

I have time, but I am looking for increasingly evil ideas to illustrate the fall without interfering with game and/or narrative structures if anyone has some tips.

If you enjoy reading novels for inspiration, you might enjoy the Brimstone Angels series of Forgotten Realms novels, written by Erin M. Evans.

The novels in this series feature a tiefling warlock that has made a pact with a devil.

 I am playing a Tiefling warlock with a feind patron and pact of the tome.  I am going with a fire based theme.  Her background is a charlatan/street magician/pick pocket.  I took a tip from xanathars guide (her parents were both human and unaware of their infernal bloodline until Cinder was born) her parents were ashamed of her and dumped her at an "orphanage for the undesirable races".  She set out on her own as a teen and learned the basics of slight of hand and street magic.  One night at her back alley camp she heard a voice whispering from her camp fire, offering her both power and a bit of razzle dazzle for her street performances, but for a price.  

I'm building her primarily as a blaster (fireball, hunger of hadar, eldricth blast with agonizing and repelling blast) with some escape/survivability potential (thunder step, mirror image, mage armor AC17) and battlefield control (hunger of hadar, hypnotic pattern).  

Anyway, on to my question.  She does not know much about her patron, other than it's proclivity for fire and she suspects it is interested in her performance, manipulation and theivery abilities.  But I as a player, don't know much either.  I don't want a ton of information but if anyone has a fiend that comes to mind that I could point my DM toward, that would be very helpful.