In my homebrew Feywild campaign, some powerful fey can use a spell that is often the result of fey deal, or a curse put upon someone by a fey. This is a very high level of control spell. Its not the same as charm person or dominate person. The fey could not directly control the targets actions, making them do specific actions. Instead, the fey are all about emotions, so they want to control the target's emotions. They can change intensity of emotion, how the target feels towards people and things. For example a target could suddenly lose their love for a loved one, or vice versa, be forced to love one they've never loved. This also controls the target's desires and fears to an extent. It would also be cool to have this serve as a scrying spell on the target too.

Its not my intention to use this on the player characters. First off I can see how this spell could be really messed up. Instead I have some npcs planned to already be under this spells effect, and one that could depending on party actions. It will be possibly used on them if they mess up really really badly. I also might make it a late game reward depending on their actions.

This spell would last until the fey is killed, or the original deal or curse nullified in some way. Otherwise I've never really homebrewed a spell before so I really don't know much about mechanics or balancing.

I'm sure you're aware that Calm Emotions can eliminate certain conditions and emotions. It sounds like you'd like your spell to (1) be limited to emotions (and not apply to conditions), and (2) do the reverse of Calm Emotions to a subject's emotions, i.e., create a new emotion (rather than eliminate the emotion).

There is a 3e spell call Emotion, which sounds similar to what you're talking about. Maybe to use 5e mechanics, you can stipulate that the spell changes a creature's Attitude about another creature or another thing.

You don't need specific mechanics or HB for this. This is a narrative ability which you can simply define by how it alters the behaviour of the NPCs and how the PCs can end it. In fact, I would explicitly warn you NOT to define it as a "spell" or a "curse" because 5e has 3rd level spells which can easily dispel any spell or remove any curse which players will expect to be able to use to counter/end this effect if you define it as a spell or a curse. 

Quote from Agilemind >>

You don't need specific mechanics or HB for this. This is a narrative ability which you can simply define by how it alters the behaviour of the NPCs and how the PCs can end it. In fact, I would explicitly warn you NOT to define it as a "spell" or a "curse" because 5e has 3rd level spells which can easily dispel any spell or remove any curse which players will expect to be able to use to counter/end this effect if you define it as a spell or a curse. 

 Thank you, I was kind of thinking that too after a while. And thanks to the previous guy for the suggestion. I might use that spell in some form possibly.

Also, I'm not worried about labeling it as a curse. I've already said in passing to this same group of players that the spell remove curse will not exist in my campaign. It spoils too much fun lol. And its never made sense how powerful it is to me. Plus this same group is currently running a game where the dm homebrewed the remove curse spell to give it more of a check to remove. So this group is used to that spell being nerfed lol.