So here are the invocations I have been working on, all of which are in the "first draft" stage. I welcome feedback.
You have a good start. One thing to consider is to make the effects as simple as quick as possible to resolve. My biggest critiques will probably be the ones which will be annoying to resolve in play (over and over).
BLAZING BONFIRE
Prerequisite: Create bonfire cantrip, 5th level
When you cast create bonfire, it creates an initial burst of heat that blasts nearby creatures. Each creature within 5' of the bonfire when it forms must make a Con save or take 1d4 fire damage.
This is actually worse than the base cantrip because it cannot be controlled. It does little damage and doesn't scale. I don't think the concept works well.
BLINDING FLAMES
Prerequisite: Create bonfire cantrip
When a creature fails its save against your create bonfire cantrip by 4 or more, it is also blinded until the start of its next turn.
This is a very powerful effect. Invocations should be powerful but even with the save clause, having advantage to attacks against the creature is pretty big.
Isn't the standard additional effect for failing saves in 5e 'by 5 or more' not '4'?
What if instead the Bonfire created billowing smoke? This is a very Warlock thing and the effect would be to have that space be heavily obscured. If the creature was in the area they would be blinded but other creatures would have disadvantage to hit them too.
The result would be to make the cantrip much more powerful as an area control ability.
BRILLIANT FLAMES
Prerequisite: Create bonfire cantrip
When you use the create bonfire cantrip, you can choose to have it deal radiant damage instead of fire damage.
Okay, can we give it +1d8 dmg vs Fiends and Undead too?
Just changing a damage type is quite weak for an invocation.
FELLING FROST
Prerequisite: Ray of frost cantrip
When you hit a creature with ray of frost, it also falls prone.
This is too good. Advantage against the creature for (melee) allies until the creature's next turn is too much.
FREEZING BURST
Prerequisite: Ray of frost cantrip
When you hit a creature with ray of frost, each other creature within 5' of it must make a Con save against your warlock spell save DC or have its speed reduced by 10' until the end of your next turn.
This doesn't pass my 'keep it simple and fast' rule. That's a lot of saves to be making and things to keep track of.
HINDERING FROST
Prerequisite: Ray of frost cantrip
When you hit a creature with ray of frost, the next time it voluntarily moves before the end of its next turn, it takes damage equal to your Charisma bonus.
This is very weak.
LINGERING TOUCH
Prerequisite: Chill touch cantrip
When you hit a creature with chill touch, at the start of that creature's next turn, it takes necrotic damage equal to your Charisma bonus.
This is weak. Granted, I believe that the original design for Agonizing Blast was to have it activate only once and then it wasn't caught in the final edit (likely to Eldritch Blast being changed to multiple blasts or something).
Still, no one is going to take a worse cantrip like Chill Touch and couple it with a much worse invocation than Agonizing Blast.
NECROTIC CURSE
Prerequisite: Chill touch cantrip, 12th level
When you hit a creature with chill touch, you leave a lingering necrotic curse on that creature. This curse ends after it is triggered or the target completes a long rest. It also ends if the creature is subject to a dispel magic or to an effect that will remove a curse.
The curse triggers when the target regains hit points. At that point, it must make a Con save or becoming poisoned for 1 hour. It can repeat the save at the end of every 10 minutes, ending the effect on a success.
This is:
a) A lot to keep track of
b) Is not likely to come up in play often (which is also the main problem with Chill Touch in the first place).
WITHERING CHILL
Prerequisite: Chill touch cantrip
When you hit a creature with chill touch, that creature must also make a Con save. If it fails, until the end of its next turn, the creature suffers a penalty to weapon damage rolls equal to your Charisma bonus (to a minimum of 1 point).
No save. Just make it happen. Compare it to Frostbite. It would be quite good against creatures with many attacks but quite weak against ones with fewer. Basically an inverse Frostbite/Vicious Mockery.
ENVENOMED FANGS
Prerequisite: Poison spray cantrip
When you cast poison spray, you can choose to have the spell grant you fangs until the end of your turn instead of having its normal effect. If you do so, you can bite a creature within your reach as a bonus action. Make a melee spell attack; if you hit, the target takes 1d4 + your Charisma bonus piercing damage plus the damage dealt by your poison spray cantrip.
Okay.
NAUSEATING POISON
Prerequisite: Poison spray cantrip
When a creature takes damage from your poison spray, it can't gain advantage or take reactions until the end of your next turn.
This is a weird mechanic and it's not powerful enough for an invocation. Compare to Shocking Grasp.
LINGERING POISON
Prerequisite: Poison spray cantrip
When a creature takes damage from your poison spray, it must make a Con save. If it fails, it is poisoned until the end of its next turn.
How about no save involved but the creature has disadvantage on the next attack it makes until the end of its next turn? So Poison Spray becomes a high damage Frostbite/Vicious Mockery.
No save means it is faster to resolve.
POISON SPIT
Prerequisite: Poison spray cantrip
When you cast poison spray, the range is 30'. If the target makes its save, you can choose to force a random creature within 5' of it to make a Con save or be affected by your poison spray instead.
Choosing a random creature takes too much time.
I don't see the theme here either.
I like an invocation to increase its range to 30'. Perhaps think about adding that to another one.
POWER OF THE STORM
Prerequisite: Thunderclap cantrip
When a creature fails its save against your thunderclap cantrip, it also takes lightning damage equal to your Charisma bonus.
Too weak. Also not interesting.
FORCEFUL THUNDERCLAP
Prerequisite: Thunderclap cantrip
When a creature fails its save against your thunderclap cantrip, it is also pushed 5' directly away from you.
Given that Thunderclap is niche this is rather weak.
DEAFENING CLAP
Prerequisite: Thunderclap cantrip
When a creature fails its save against your thunderclap cantrip, it is also deafened until the end of its next turn.
This is extremely weak. Add it into another one.
Maybe combine Forceful, Deafening, and the ability to choose targets all into one invocation.
QUICK STRIKE
Prerequisite: True strike cantrip
When you use the true strike cantrip, it has a casting time of 1 bonus action.
Okay. Still a little weak.
CLEAVING STRIKE
Prerequisite: True strike cantrip
If the attack affected by your true strike drops a creature to 0 hit points or scores a critical hit, you can make one weapon attack as a bonus action.
This is very weak.
DEADLY STRIKE
Prerequisite: True strike cantrip
When you use the true strike cantrip, the affected attack scores a critical hit on a 19-20 and increases the severity of critical hits caused by its affected attack by 2d6.
You could add the increased crit range to Quick Strike.
STAGGERING FALL
Prerequisite: Fell cantrip
When a creature fails its save against your fell cantrip, until the end of its next turn, it must spend all of its movement to stand up from prone.
I don't know what this is.
THE CLOCK STRIKES MIDNIGHT
Prerequisite: Toll the dead cantrip
After a creature takes damage from your toll the dead cantrip, if it has a number of hit points equal to your warlock level or fewer, it drops to 0 hit points.
Weak. Compare to invocations which add damage.
DYING TOLL
Prerequisite: Toll the dead cantrip
When you cast toll the dead, instead of its normal effects, you can choose to use it to force each dying creature within range to make three death saves.
This is extremely weak. How often will this matter?
BLOODY TOLL
Prerequisite: Toll the dead cantrip
If a bloodied creature fails its save against your toll the dead cantrip, it takes an extra 1d6 damage per die of damage your cantrip deals.
What is 'bloodied'? Half HP?
This is weak.
A Toll the Dead 'cleave' might be a fun idea. If you drop a creature to 0 you get to cast the spell again. Would probably need something more too.
Hope this feedback was helpful.
