I have been kicking around the idea of a person who's devotion to a being made them it's preferred vessel the they get killed, need to hide, are trying to enter this world etc. This would make you the living embodiment of this creature that is granting you what powers it has in this limited state and over time your souls become closer and you can better channel their power. This sounds really cool to play and could work for any of the patrons in the book I think. Race is the main problem I'm having. Trifling makes sense for someone inhabited by a demon, but there are not amazing races for those who should have additional fey or old one aspects naturally. Eladrin works ok for fey but is too normal. Any suggestions?

Consider a human who is beginning to fuse with an elf... perhaps they end up resembling a half-elf, so you build them as a half-elf and just say they're human.

Oh! Thats a cool idea! Maybe at level 5 they can be transformed enough to turn from a human to half elf. Thanks!

My Warlock Erizel was a regular human, who forged a pact with a mysterious Archfey. Its only after lots of experience as a Tome Pact Warlock, that she discovers that her patron is her Past/Future self. In our home campaign there will come a time where Erizel performs the rituals of Apotheosis and Ascension, which will transform her into a Fey then Archfey.

I love your living embodiment idea.

Archfey- Human (Variant- Magic Initiate/Spell Sniper/Lucky or other interesting feat) or Half Elf

Fiend- Tiefling or Human (Variant)

Great Old One- Human (Variant), Gnome (Deep Gnomes), Half Elf(Drow Descent), Half Orc, Tiefling (Devils Tongue)

Those are great racal ideas! Also that is some really cool background for a warlock.

Thanks, my Warlock Erizel is really fun to play, she's also spy whose cover is a 'Travelling Librarian' and may or may not be a Harper.

Another idea, I had is that maybe the race is irrelevant or a contrast- An Archfey 'Hag' who is in love with a Half Orc or Tiefling, a Fiend who has made a pact with an innocent seeming Gnome/Halfling or an unconventional Dwarf. A Great Old One from beyond the Outer Darkness who haunts the dreams or whispers to a pragmatic Dwarf stonemason or inspires a Dragonborn with visions of past and future Dragonborn glory.

Darkfiddlers idea of a human transforming into an Elf is cool.  You can even do that in an Adventurers League campaign, level 1 your a human, level 3 Half Elf and later change your race to Elf. Obviously your skill set features, attributes and proficiencies will change but your equipment remains the same.

Hmmm. Mabie you are right. I think mabie there are other ways besides race that could make a character have a one with patron flavor. Mabie some feature that is equally good as bad. Mabie the power to summon former allies but to get them on your side you have to convince them you are your patron. And also they might have betrayed their patron. Once per week after level 5 sounds about right. Or mabie your patron is not wanted so they can sometimes try and take over your body. That's used often though. So many posiblilitys! 

Oooo! One last thing! Mabie you can have difernt forms that change race and connection with patron! Pure you form, where you are your natural race,taken form, where you don't have control but your patron can better Chanel their power but when you come out of it 1 level of exoustion, and than mixed form, when you are the perfect host for your patron and become equally them and yourself just available at high levels and very powerful but with some bad affect. 

Might have to write a whole guide about this.

I have an undying pact warlock that is possessed by his dead necromancer ancestor. The ancestor is teaching the warlock to be a necromancer so that he can eventually trick the warlock into casting a ritual that will let him take over the warlock's body permanently. 

Me, I made Aquilon, a sorcerer storm sorcery genasi of elemental air who is a child of a noble djinni. After 6 lvl with this class, I multiclass it with warlock, so the reason is that Aquilon is worthy of receving the power of his father. But he died by committing suicide by carrying an enemy, a powerfull, in his vessel that his father gave him. My DM has to accept this.

I had kind of a similar idea. My DM is saying that she would eventually like to run a monster campaign. I remember hearing somewhere that in 5e Death Knights do not have to be based on fallen paladins and instead can be any class. I don't know if that's true or not, I don't have the books that would reference this but if it is the case my idea was a Hexblade Death Knight. Basically, a hexblade lock, sold his soul to his weapon for power bit by bit, and eventually tried to kill himself to renege on the deal, the weapon held on, refusing to relinquish it's claim. Now they are bound to each other, him trapped in a state of undeath, his soul trapped in the blade he wields, his will has mastered the usage of the weapon but cannot force it's sentience to release his soul.

Four years ago, the original poster wanted to know what Race would be the most fun to play for a Pact of the ArchFey Warlock.  I say that their Patron should be Titania of the Summer Court, and their Race should be Saytr, from the Mythic Odysseys of Theros, pg. 24

Saytrs are traditionally male, so part of the fun with having the Queen of the Summer Court inside your head would be the slow transformation into something female.  Step by step, they lose their male appearance, until at last they look just like an Elf of whatever Race seems most interesting and fits the setting.  If the player doesn't want their character to change, use Oberon, Titania's Consort.

An interesting idea. Diavalo/doppio type thing? Maybe use this. It is a dark gift from Van Richten's guide to Ravenloft. This is not available as a choice unless your DM lets you do it, so keep that in mind.

Symbiotic Being

A second being resides within your body along with you. It manifests physically somewhere on you and relies on you for its survival. At the same time, it offers you knowledge and other assistance, either of its own volition or because of your intertwined nature. The symbiote has its own personality and agenda, which might be at odds with yours. If you ignore the symbiote’s desires, it might strive to interfere with you. There is no way to be rid of the symbiote while you have this Dark Gift. Even if you go to drastic lengths, such as severing the part of your body where the symbiote resides, it reappears on your body in another location at the next midnight.

Roll on or choose an option from the Symbiotic Nature table to determine what sort of symbiotic being shares your body. Additionally, you gain the traits that follow.

Symbiotic Nature

Entwined Existence. Your symbiote is a separate entity with its own physical form bound to yours. It isn’t a separate creature and relies on you to survive. It has Intelligence, Wisdom, and Charisma scores. The DM sets the symbiote’s abilities or determines them randomly (roll 4d6 for each score, ignoring the lowest roll and totaling the rest). The symbiote can see and hear using your senses.

The symbiote speaks, reads, and understands two languages; one that you speak, as well as one appropriate to its nature. Choose one of the following skills: Arcana, Deception, History, Intimidation, Insight, Investigation, Nature, Religion, Perception, or Persuasion. You gain proficiency in that skill if you don’t already have it, representing the symbiote’s counsel and guidance.

If you die, so does your symbiote. If you are subsequently returned to life, your symbiote revives as well.

Sustained Symbiosis. Your symbiote has a vested interest in your survival and takes steps to ensure it. When you fail a saving throw, you can choose to have your symbiote expend one of your Hit Dice to roll it and add the number rolled to the saving throw, potentially turning the failure into a success. If it uses this feature on a death saving throw, you succeed on the save and regain 1 hit point regardless of the number rolled on the d20.

Once you succeed on a saving throw due to this trait, the symbiote can’t use it again until you finish a long rest.

Symbiotic Agenda. Your symbiote has an agenda that drives it, and it expects you to assist it in achieving those goals. How permissive or patient it is in resolving its agenda depends on its personality (work with your DM to determine these details). If you have an opportunity to advance its agenda and don’t act on it, the symbiote can try to force your hand. You must succeed on a Charisma saving throw (DC 12 + the symbiote’s Charisma modifier) or be charmed by the symbiote for 1d12 hours. While charmed, you must try to follow the symbiote’s commands. If you take damage that is not self-inflicted, you can repeat the saving throw, ending the effect on a success.

Roll or choose from the Symbiotic Agenda table to determine the symbiote’s goal, working with your DM to detail the specifics or create another agenda.

Symbiotic Agenda