You might be wondering how this could happen apart from a contrived scenario of casting a spell like cause fear on yourself (which is allowed). Turns out, it can happen by interacting with the local wildlife of Icewind Dale.
The Crag Cat has this ability:
Spell Turning. The cat has advantage on saving throws against any spell that targets only the cat (not an area). If the cat’s saving throw succeeds and the spell is of 7th level or lower, the spell has no effect on the cat and instead targets the caster.
So I cast cause fear on a Crag Cat, it passes the save, and then I fail on my save. I become the target of my own cause fear, which means I am now frightened of myself.
The frightened condition says:
- A frightened creature has disadvantage on ability checks and attack rolls while the source of its fear is within line of sight.
- The creature can't willingly move closer to the source of its fear.
Do either of these conditions apply in some way while I am frightened of myself?
You might be thinking, "why not just drop concentration and end the effect?" Right, that would work, if the caster thought to do that. When this scenario played out in my game, the player whose character was frightened of himself was so worked up about his player being frightened of himself he didn't even think to drop concentration, which I thought was a great narrative expression of the effect. I ruled on the fly that he used all of his movement on his turn to run about wildly, trying to get away from himself, opting to make a quick ruling without spending much time thinking to preserve the tension of the situation. Now that I have had time to think about it, I'm not sure what the correct ruling would be.
Best Answer
Both bullet points take effect, but the second one is vacuous.
In its title and in its body text, the question explicitly asks about what happens if you are frightened of yourself. This is exactly what I am going to answer. Whether casting the cause fear spell on yourself (intentionally or by Spell Turning) becomes a non-issue because being frightened would cause you to lose your concentration on the spell immediately, would be a different question.
Considering the first bullet point: The crucial question here is: is a creature considered to be within its own line of sight? The only precise definition of line of sight is given in the context of the optional rule of playing on a grid (DMG, page 251, emphasis mine):
The word another seems to prohibit using this definition in order to decide whether there is line of sight within the same space - which would be the relevant case in this scenario.
In the absence of a general mechanical definition of line of sight, we will have to go with its plain English meaning. I just tried around a bit, and I find it difficult to move my body naturally without seeing any part of it (or the clothes I'm wearing), unless I close my eyes. In fact, no matter where I look, my nose and part of my beard are always in my peripheral vision. It takes active effort not to see any part of my body, so I'd find it rather unnatural not to consider a creature to be within its own line of sight. Being the source of its own frightened condition, it should thus be subject to the consequences of the first bullet point.
Note that this interpretation of line of sight is consistent with the rules on Targeting Yourself with a spell (PHB, page 201), as is discussed e.g. here, here, or here (thanks to NautArch for referring me to these questions).
Considering the second bullet point: Since there is no additional condition in this bullet point, it definitely takes effect. The only remaining question is whether it actually makes a difference. "Moving closer to something" is obviously supposed to mean "reducing the physical distance to something", not some metaphorical or spiritual distance, or whatever other notion one could come up with. The rules on movement in combat say:
This seems to equate the distance to something with the movement it takes to get there. Since staying exactly where you are costs no movement at all, the distance to yourself is therefore 0. As a negative distance is impossible, no conceivable move could bring you closer to yourself than you are at the moment. Consequently, you cannot move closer to yourself - regardless of whether you are frightened of yourself or not.
However: That said, I think your ruling sounds much more reasonable, more in line with the idea of the frightened condition, and simply more fun than sticking to this strict reading of the rules.