The major draw for the original blink spell is that, for a squishy Wizard, the character is not around to get attacked. But the downside is that the character is not around to attack someone else or be useful. This spell doesn't have a downside. You're either normal, or unstoppable.
The character has removed their body just enough so that, about 50% of the time:
- (a) All attacks are at disadvantage
- (b) ALL DAMAGE is resisted (halved)
- (c) Damage you do take cannot be critical
- (d) If all of that somehow brings you down to 0 hp, you instantly pop back up to 1 hp
- (e) You cannot be killed instantly via spells/effects that don't use hp
- (f) It doesn't require concentration, so you could also have another damage mitigation spell running in parallel
And I don't know what to make of the spell having a duration of 1 minute, but "you gain the following benefits until the start of your next turn". Does it last 1 minute or 1 round? Since you also mention, "you instead drop to 1 hit point, and the spell ends" as one of the benefits, I'll assume it was a bad cut and paste go with this lasting a whole minute.
And after all that, 100% of the time, the character can attack normally, cast spells, move objects, take reactions, and otherwise be completely functional.
All of this for the cost of a 3rd-level spell slot (and I guess being the right race). Why wouldn't every Shadar Kai have this memorized/known?
Let's examine some of these things:
(a) - Looking at the beasts attached to shadows (shadow, shadow mastiff, shadow spirit, shadow demon, etc), none of them naturally impose disadvantage on attacks. Some can hide in shadows to give them advantage and an attacker disadvantage, but that still take effort on their part (usually a bonus action). Here the caster gets it for free. Additionally, many of them suffer when in direct sunlight. Not a problem with this spell.
(b) - With the exception of the "Blessing of the Raven Queen," I can't think of a single spell, feat, or feature that allows resistance to all damage. Generally, these kinds of effects will give resistance to a bunch of different damage types, or more commonly it's against non-magical attacks. But across the board, negation as an effect is less than 1%. And even for this one case, it's only for a single round, not 50% of the combat.
(c) - Once again, looking at other shadow beasts, all of them are still susceptible to critical attacks. There are objects, creatures, and what have you that cannot suffer a critical hit, but having it for the cost of a low-level spell makes this a bit too much. For perspective, at this level, a Monk can critically punch a ghost to death.
(d) and (e) mimic the effects of death ward, a 4th-level spell, just with a shorter duration but now for classes that could not normally cast death ward. But you would only cast this when you need it, as opposed to a backup in the morning that may or may not come into play.
(f) - As pointed out above, this doesn't take concentration. I realize that the original blink didn't either, but it also had drawbacks. But with this spell, the character is in the thick of it, expecting to be hit (albeit at disadvantage, non-critically, and automatically halved).
My suggestions for the spell
(a) - You can either give the character advantage in dim light or darkness, or enemies disadvantage in dim light or darknesss. One or the other, not both, and not while standing in the middle of a brightly lit area.
(b) Only resist bludgeoning, piercing, and slashing from nonmagical attacks. Remember, you're only partially on the Shadow Plane. The rest of you can still burn, freeze, be electrocuted, etc.
(c) Sorry, almost anything can be critically hit. Need to drop this.
(d) While it doesn't really fit with the theme of "shadow", I'm fine coming back from 0 hp one time. Ending the spell at this point is also pretty common.
(e) You have access to Counterspell, use it. You might be able to get away with saying, if the spell/effect has a saving through, the character gets advantage on the saving throw.
(f) With the above changes, I'm not as concerned about not being a concentration spell. Since it can end if the character is knocked down to 0 hp it's not a complete "set it and forget it" spell. Plus in your use case of a Bladesinger, they will have used their Bonus Action to activate Bladesong, and an Action to cast this, meaning they will have already lost one round of combat.
Final spell would look something like
3rd level transmutation
Casting Time: 1 action
Components: V, S
Duration: 1 minute
Classes: Sorcerer, Wizard (Shadar Kai specific)
Roll a d20 at the end of each of your turns for the duration of the spell. On a roll of 11 or higher, you partially vanish from your current plane of existence and partially appear in the Shadow Plane (the spell fails, and the casting is wasted if you were already on that plane). During that time, you appear ghostly and translucent. You can dismiss this spell as an action.
For the duration of this spell, while on the Shadow Plane, the target gains the following benefits:
- In areas of dim light or darkness, creatures have disadvantage on attack rolls against the target.
- You have resistance against bludgeoning, piercing, and slashing damage from non-magical weapons.
- The first time you would drop to 0 hit points as a result of taking damage, you instead drop to 1 hit point, and the spell ends.
- If you are subjected to an effect that allows a saving throw and would kill you instantaneously without dealing damage, you gain advantage on that saving throw.