The Thaumaturgy cantrip allows you to alter the appearance of your eyes, though not your entire face.

One of the effects of Thaumaturgy is to

alter the appearance of your eyes for 1 minute.

You can dismiss the effect with an action if 1 minute is too long for you. If that's enough, it could work. Thaumaturgy is not a Warlock cantrip, but you can gain access to it through the Pact of the Tome, the Magic Initiate feat, or by multiclassing to take a level of Cleric.

Alternatively,

The Mask of Many Faces Eldritch Invocation could suit your needs.

You would need at least 2 levels of Warlock to get the Invocation, but with it

You can cast disguise self at will, without expending a spell slot.

Disguise Self lets you change your appearance in many ways, but you could choose to only change your face. It takes an action to cast and can last up to an hour, but you can use your action to dismiss it.

A race can

Take a look at the Changeling race (from Wayfinder's Guide to Eberron, which is still technically UA content).

From the PHB:

Change Appearance

As an action, you can transform your appearance or revert to your natural form. You can’t duplicate the appearance of a creature you’ve never seen, and you revert to your natural form if you die.

You decide what you look like, including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. You can make yourself appear as a member of another race, though none of your game statistics change. You also can’t appear as a creature of a different size than you, and your basic shape stays the same; if you’re bipedal, you can’t use this trait to become quadrupedal, for instance. Your clothing and other equipment don’t change in appearance, size, or shape to match your new form, requiring you to keep a few extra outfits on hand to make the most compelling disguise possible.

Even to the most astute observers, your ruse is usually indiscernible. If you rouse suspicion, or if a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.

Minor Illusion

If you do want a cantrip, the cantrip that comes to mind is Minor Illusion, however this takes an action to cast and another action to end the illusion, which would not exactly be "a flash".

Mask of Many Faces

This warlock invocation allows you to cast Disguise Self at will - allowing to make yourself "look different". I don't see any reason why this wouldn't extend to your lucifer-like modifications.

It does give the opportunity for the victim to make a check to see if they know what's going on, and physical inspection is a big no-no, but this is an easy way to achieve the effect you want without having to worry about spell slots.

Arguably, Shape Water could do this

This would require you to keep some water in a color and opacity of your choice in a container on your person at all times. But then, you could cast Shape Water, which allows the following effects:

• You move the water up to 5 feet in any direction. This movement doesn't cause damage.




• You cause the water to form into simple shapes and animate. This change lasts for 1 hour.

I would argue that a jack-o'-lantern quality face would be scary and count as "simple shapes". You'd have a weird "mask" of moving water, which you could then dismiss by moving it again.

Note: Shape Water is a Druid cantrip that is not naturally available to Warlocks. You can get around this with either the Magic Initiate feat, taking one level of druid, or using the Pact of the Tome Warlock feature.

If magic items are OK ("any other feature"): Hat of Disguise

The Hat of Disguise is an uncommon item that requires attunement. Whether or not one drops or is available to you is in the DM's hands. It does this for you:

While wearing this hat, you can use an action to cast the disguise self spell from it at will. The spell ends if the hat is removed. (Hat of Disguise; SRD p. 225)

The disguise self spell meets your needs, and (with the item) will be castable at will like a cantrip (extracts from SRD p. 135):

Duration: 1 hour {more than you need}

You make yourself look different until the spell ends or until you use your action to dismiss it...the extent of the illusion is up to you. {snip} To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC.

In the amount of time you are looking to apply the disguise, that Int check is likely moot.

At higher levels, use Warlock Invocation Master of Myriad forms

That invocation let's you cast Alter Self at will. While it requires level 15, that is, quite a lot, it's not a disguise, you literally take on the shape you wish.

Recommendation: try talking to DM into allowing the use of this invocation at level 5-7, since the spell is only 2nd level and doesn't really give any combat advantages (you can negotiate locking out the Natural Weapons part of it until intended at lvl 15).

As another option, if you're only interested in roleplaying the intimidation, and not interested in getting a mechanical boon on top of it, I would look into trinkets.

Any character is allowed a free trinket, and Curse of Strahd provides Gothic Trinkets table that contains some quite magical items. If

a little black book that records your dreams, and yours alone, when you sleep

can be a trinket (and an actually useful thing, in case of prophetic dreams), then I don't see why "A ring, that makes your visage horrific for a second when you press the stone in it" can't be an option.

Unfortunately PHB doesn't really mention "you can think up your own trinket", so some will argue that it's illegal, but really, nothing is illegal if DM allows it. Work with your DM on this.

Using thaumaturgy, and potentially minor illusion

With the thaumaturgy spell, there is a RAW way to accomplish at least part of what you want:

You manifest a minor wonder, a sign of supernatural power, within range. You create one of the following magical effects within range: [...]

• You alter the appearance of your eyes for 1 minute.

Depending on how your DM will handle illusions, minor illusion may also be a solution if you are willing to stand REALLY still (the illusion can't move) with the illusion just barely in front of your face.

There is also a related topic on this site (Can you create an illusion of empty space?) about not being able to use minor illusion to "delete" sections of objects as part of the illusion, or create empty space. This may apply somewhat if you were trying to morph your face in such a way that you would want to shrink parts or make parts disappear.

The minor illusion spell description says:

You create a sound or an image of an object within range that lasts
for the duration. The illusion also ends if you dismiss it as an
action or cast this spell again.

[...]

If you create an image of an object - such as a chair, muddy
footprints, or a small chest - it must be no larger than a 5-foot
cube. The image can't create sound, light, smell, or any other sensory
effect. Physical interaction with the image reveals it to be an
illusion, because things can pass through it.

If a creature uses its action to examine the sound or image, the
creature can determine that it is an illusion with a successful
Intelligence (Investigation) check against your spell save DC. If a
creature discerns the Illusion for what it is, the illusion becomes
faint to the creature.

Personally, I would just flavor this. Whenever you make a Charisma (Intimidation) check, just describe that your face changes appearance momentarily. As a DM, I wouldn't make you actually go and expend build resources for something that won't have an actual mechanical effect.

(On the other hand, if you're expecting a mechanical effect, spells, effects and cantrips do what they say they do, so you're kind of stuck.)