In a recent post, I went into some detail breaking down what I see as problems in the design of the warlock class. Today I'm suggesting a few new invocations. The invocations that allow the warlock to cast a spell, at the cost of a warlock slot, once per long rest... we can agree that that's paying a very limited resource for a very limited benefit, right? I linked it last time, and I'll do so again: this optimization thread on the Wizards forums is a great resource to me, but not in the light that the author probably imagined.
As an action, sacrifice a number of hit dice equal to the level of your Pact Magic slots to refresh a Pact Magic slot. Once you use this ability, you may not do so again until you complete a long rest (alternately: until you use the Recuperate downtime action).
Notes: This is intended to be a desperation play, and a lesser version of Eldritch Master.
As an action, you may target one undead creature you can see within 30 feet. The target must make a Wisdom saving throw against your spell DC. On a failed save, the target must obey your commands for the next 24 hours. An undead creature whose challenge rating is equal to or greater than your level is immune to this effect. Once you use this ability, you may not do so again until you complete a long rest.
Notes: Based, obviously enough, on the Oathbreaker paladin's Command Undead ability, but once per long rest rather than once per short rest, as a Channel Divinity. I've been meaning to write up the Grim Reaper as a warlock patron, and this feeds into that idea.
As a reaction, whenever you or someone within 5 feet of you rolls a saving throw against a magical effect, you may add 1d6 to the result. Declare this ability after the saving throw is rolled but before the DM declares the effect.
Notes: I thought about making this a cantrip. As it is, I think this is very good, though for the really troublesome area effects, you're only aiding yourself or one ally. If it's too much as is, dial it down to one use per short or long rest. What I like about this idea is that it gives the warlock a way to be more of a team player - as it is, they have very few abilities that intersect with allies, other than worrying about excessively close air support.
Labyrinth Walker In some settings, this may be restricted to warlocks with Archfey patrons.
You are experienced in the mental discipline and mysteries of walking a labyrinth. You gain advantage on all saving throws against the charmed condition, the frightened condition, and the spells compulsion, confusion, and maze.
Notes: Considering how many of these effects are Wisdom saves, and that the warlock is proficient in same, it's not immunity, but it's a meaningful resistance. Of course, not many warlocks will have a great Wisdom score.
Master of Maledictions
As a bonus action, you place a curse upon a creature you can see within 90 feet. This requires your Concentration and lasts up to one hour. Until the spell ends, you deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack. Also, choose one ability when you use this effect. The target has disadvantage on ability checks made with the chosen ability. This effect does not stack with hex. When you have used this effect a number of times equal to your Charisma modifier, you may not use it again until you complete a long rest. A remove curse cast on the target ends this effect early.
Notes: I liked how the 4e warlock's curse was cheap-as-free. Hex is really good, but for it to take one of your two Pact Magic slots for a whole battle... and for the game to "expect" two battles per short rest... I'd like warlocks to feel like hex isn't all they have the available slots to cast before 11th level. This invocation adds a number of additional targets equal to your Charisma modifier that you can hex in a day, but without transferring the hex when the target dies.
You may spend 10 minutes to inscribe a rune of trial and peril. If you pass its tests, it strengthens you for battle yet to come; if you fail, it takes a dreadful toll. Whatever the outcome, you may not inscribe the Sigil Perilous more than once per week. Use one of the two models below, depending on what suits your campaign's style or the needs of the moment.
Riddles in the Dark. Through direct contact with your patron or one of your patron's highest-ranking servants, you must show your worthiness. The Archfey favor riddles and tests of loyalty, the Fiends favor demonstrations of the corruption deep within your soul (and tests of loyalty), and the Great Old Ones favor mind-shattering pseudo-mathematics. (I don't know how a DM might test a PC on that last one. Let's say I'm leaving that as an exercise for the reader. Ahem.) To pass, warlocks of levels 1-5 must answer at least one question correctly out of five (or whatever amounts to 20%); warlocks of levels 6-11, two or 40%; warlocks of levels 12-17, three or 60%; and warlocks of 18th level and above must answer four or 80% correctly.
Let the Dice Be Your Oracle. Your patron tests your Charisma, and one ability score appropriate to your patron's nature and your own bond with that patron. Archfey typically test Wisdom (or Insight), Fiends, Constitution; and Great Old Ones, Intelligence (or Arcana). Roll five ability checks, of increasing difficulty; the DM decides which abilities your patron tests, and these are often different from one trial to the next. The ability check DCs do not increase according to your level. To pass, warlocks of level 1-5 must succeed two checks; warlocks of level 6-11, three; warlocks of level 12-17, four; warlocks of 18th level and above must succeed all five checks to pass the test.
Success! If you succeed, you may use each feature that your patron grants you one additional time before taking the required rest. (That is, the class features specific to your Patron.) This benefit fades when you next take a long rest.
Failure... If you fail, for the next 24 hours, you make all spell attacks with disadvantage, and you grant advantage on all saving throws that you force opponents to make.
Notes: This is obviously complicated to resolve, but I liked the name The Sigil Perilous, and this is what I came up with for that to do. This invocation emphasizes the warlock's relationship with the patron, and might just as easily be part of an antagonistic relationship as a more cooperative one. It's definitely not for every group, in any case.
Tendrils of Ruination Prerequisite: 15th level
As a bonus action or reaction instead of an action, you may cast arms of Hadar as a 1st-level spell without expending a spell slot, even if you to not know the spell. If you do know the spell and spend a warlock spell slot, you may instead cast it as any spell level up to 5th.
Notes: So there's this really gross, unique warlock spell at first level, right? It's pretty situational, because it's not so good at friend-or-foe identification. With just 13 spells known at 15th level, and just 15 at 20th, I'm guessing it gets de-prioritized out of a lot of builds. Therefore I'm curious as to whether shifting it to a bonus action or reaction would make it appealing in high-level play. I find this really hard to judge, since I haven't personally played or run any high-level 5e yet.
Weaver of Twilight Prerequisite: 3rd level
You can cast web once using a warlock spell slot. Once you do so, you may not do so again until you complete a long rest. When you reach 9th level, this invocation no longer costs a warlock spell slot, but you still must complete a long rest in between uses.
Notes: An invocation that granted you a second-level spell, even a pretty good one, is not an invocation that you're keeping once more of the daily-spell options open up. I feel like most of those daily-spell options would be a lot more compelling if, at (prereq level + X), they no longer cost a Pact Magic slot, but became simple once-per-long-rest effects. Thief of five fates springs to mind here. After all, at the highest levels, you have eight invocation slots, but every invocation on the list is in the running. It might help if some of the early invocation options grew with you, even slightly.