One of the strength of spellcasting is that you have a large list of powers to select one. So maybe the feature should also be designed to have several options, but themed differently than invocations?
But of course that also means you need to come up with more abilities.
I like the idea of curses, though I think a curse that just gives advantage/disadvantage might be a bit boring, to be honest.
Warlock's Curse
You can curse a creature that you can see as a bonus action. You can have one such curse active at a time. If you sustain a curse for a full day, it no longer counts against this limit.
When you start your turn, you gain 1 dark fate. You can have no more than x (by level) dark fates.
When a curse lasts for a full month, you can choose one of the Curse abilities you have. The effect is now automatically triggered without needing to spend dark fate.
A curse can be removed:
- By you with a bonus action (you can end it as a free action if the creature dies).
- By another Warlock as a ritual. The ritual lasts an hour for a curse that lasted less than a day, and 8 hours for a curse that lasted a week.
- With the Remove curse spell or similar
Omens of Ill Fate
Curses of ill fate trigger when your cursed creature fails an ability check, attack roll, or saving throw during its turn.
Omen of Bad Luck
When a cursed creature fails a ability check, attack roll or saving throw during its turn, it has disadvantage on its next check, roll or throw.
Omen of Insanity
The creature makes a free attack against one of its allies. If no allies are in range, it attacks itself.
Omen of Twisted Rewards
You or an ally you can see can spend a hit die and recover 1d6 additional hit points.
Curses of Revenge
Curses of Revenge trigger when a cursed creature damages a creature with an attack or spell during its turn.
Punishment of Blindness
The creature becomes blinded until the end of its next turn.
Punishment of Thorns
The creature suffers 1d6 damage.
Punishment of Regret
The creature is overcome by regret, giving it disadvantage on attacks rolls, ability checks and saving throws until the end of its next turn.
Curses of Weakness
Curses of weakness occur when you hit a cursed creature with an attack or it takes damage from one of your spells or Warlock abilities (other than curses).
Pact of the Chain: You can also trigger these curses if your familiar made the attack or cast the spell.
Weakness of the Flesh
The target takes 1dx damage.
Weakness of Resolve
The target's next attack or spell deals (or heals) only half damage.
Weakness of the Mind
The target must make a saving throw before it attempts to cast a spell or make an ability check to recall information. If it fails, it cannot retrieve the information. For spells, this means it cannot cast the spell until the end of its next turn. It can still cast other spells or use its action differently, but not the spell it originally intended.