Wrathful Smite? For a Warlock whose only gets two spell slots at a time, that spell seems a waste (it only works on 1 hit).
It will only be a 1-level dip in Warlock, so only 1 Warlock spell slot. However, I will also have slots from Arcane Trickster that I can use to cast it and this is ideal for a Rogue.
Wrathful Smite is really OP for a 1st level spell and is up there with shield as one of the most pwoerful 1st level spells in the game. The key here is it is a bonus action to use and if the enemy fails his initial save he enemy must use an action to make a Wisdom ability check to break the frightened condition.
This is way powerful, first sucking up an action to try to break it, then being an ability check means the enemy gets no proficiency bonus, gets disadvantage on the check because he is frigthened and can't use legendaries to break it (although he could use legendaries on the first save).
This is the only spell I know of that is this way. Almost every other spell is a save to break the effect, usually a save that happens without spending an action. The reason it does not get more notice is it is a Paladin spell.
Hex seems much better, both for the offensive & utility use of the spell.
Hex's main advantage because of the duration, other than that it is inferior, although I will also have Hex as well, just later in game.
The frightened condition is extremely powerful, not just because of the disadvantage it poses, but also because of the restriction to movement. If it is a melee-only enemy (as most are) and you land it, you can just back away and use missile weapons with impunity until the enemy manages to make his wisdom check with disadvantage. If it is an enemy with ranged attacks, those attacks are at disadvantage until he decides to use his action to break the frightened (and then he still probably won't succeed)
In the games I've seen the Warlock is far better off getting non-combat utility spells than combat spells like shield, wrathful smite, witch bolt and the like.
Hexblade is the only Warlock (and I believe the only non-Paladin) that can get Wrathful Smite. You can't even get it through a Feat.
I've seen that a Warlock uses leveled slots too quickly to expend them in most fights. Spamming Eldritch Blast may not be very spectacular, but it gets the job done so you can do the really interesting stuff between combats.
I won't have Eldritch Blast at all. The cantrips I will have as Warlock are Booming Blade and Magic Stone. As Rogue I will have Green Flame Blade, Mage Hand and Friends.
Magic Stone is better than Eldritch Blast on this build because I can't get sneak attack with Eldritch Blast.
I also question why the one level dip into Warlock instead of just doing Eldritch Adept
The Hexblade Warlock dip give me martial weapon proficiency and access to Wrathful Smite. Those are not available through Eldritch Adept.
Figther for the superior technique Fighting style with Quick Toss. This will let me use a bonus action to make a sneak attack and follow it up with a spell on an action. I can take this at level 6 getting it earlier than I would get a 2nd feat., and why the fighter level instead of just a martial feat?
I also plan on taking the martial adept feat at Rogue 8 which will give me a 2nd battlemaster dice and let me do this twice a day. I will also get disarming attack which has a lot of uses with mage hand legerdemain (disarm opponent, then mage hand his weapon away as a bonus action) and then a 3rd maneuver, probably the Feinting attack for when I need to both move and get advantage for a sneak attack, but I probably won't be using that much.