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I came up with a cool spell idea, but I'm unsure of what level it should be. Here's the spell:

After-Image

4th-level conjuration

Casting Time: 1 reaction, which you take when you would be hit by an attack

Range: Self

Components: V, S

Duration: Instantaneous

When you are the target of an attack roll that would hit you, this spell allows you to instead appear 5 feet away from where you were initially standing, causing the attack to pass through your after-image and deal no damage to you. This spell can only protect you from one attack, and cannot affect critical hits.

If you would need to move more than 5 feet to avoid the attack, you move the minimum distance needed to avoid the attack.

I would imagine that 4th level is high enough, but since this is a guaranteed dodge of one attack at the cost of your reaction, I'm not entirely sure. Can anyone produce a similar spell that suggests a level for this spell, or a good argument for a certain level for this spell?

Making it 5th level closely replicates the power level of an existing class feature

At 10th level (character level, not spell level), wizards of the School of Illusion get the Illusory Self ability, which is nearly identical to your spell:

Beginning at 10th level, you can create an illusory duplicate of yourself as an instant, almost instinctual reaction to danger. When a creature makes an attack roll against you, you can use your reaction to interpose the illusory duplicate between the attacker and yourself. The attack automatically misses you, then the illusion dissipates.

Once you use this feature, you can’t use it again until you finish a short or long rest.

They can do this at most once per battle, and on average 3 times a day (assuming the "standard" adventuring day with 2 short rests). If you want to put your spell on a roughly equal power level to the very similar Illusory Self feature, make it a 5th level spell. This allows a 10th-level spellcaster to use it twice a day (plus a third use for a wizard, using Arcane Recovery). Making it a spell rather than a limited-use class feature means that a character will gain more potential uses of it as they level up and gain more higher level slots. However, this shouldn't be a major problem in practice, since using higher level slots on this spell will feel like a waste to most players, considering the other uses those slots can be put towards.

If you create this spell, you should be aware that you are "stepping on the toes" of the Illusion wizard. Class features are meant to give an unique flavor to the class, and they can feel a lot less special when they can easily be duplicated by a spell, especially if the spell was available first. Of course, this is only an issue if you have an Illusion wizard in your campaign, or might possibly have one in the future.

Teleporting even a small distance is a minor additional benefit

Note that the Illusory Self ability doesn't allow the wizard to move. It just forces the attack to miss. Adding in the very short range teleport has important implications, because it takes the caster safely out of the attacker's melee range. If the attacker used all their movement to reach the caster, then the caster can run away without disengaging on their next turn. Furthermore, if the attacker has no more movement and the caster uses this on the first attack, any additional attacks the attacker could have made are wasted, or must be redirected against other targets instead. This makes your spell a bit more powerful than the Illusory Self feature. This extra power may be justified by the need to use a high level spell slot, which has an opportunity cost, unlike the class feature, whose only opportunity cost is the character's reaction for the turn.