5e’s Fear has the added mechanic of requiring the target to take the Dash action and run away, unless there is nowhere to run. This can cause some DMs to rule Oath of Conquest’s aura as allowing the target to use its action for other things. But with Sentinel’s reaction causing movement speed to drop to zero, that wouldn’t trigger until after the Dash action had been used–creating an optimizable opportunity.
My current build idea for this is a Hexblade 1 / Whispers Bard X, choosing to be Custom Lineage to grab Sentinel at level 1. This means that Cha can be maxed by 9 (rather than 13), the build is Cha-SAD, and you can augment your attacks with blade cantrips (yes, BB doesn’t synergize with either Fear* or Sentinel, but it is still a nice chunk of scaling damage). Bard also provides Command, Dissonant Whispers, Healing Word, Psychic Blades, and eventually Find Greater Steed (to get another opportunity attack).
(* If your DM counter-rules that Fear, Command, Dissonant Whispers, etc should proc Booming Blade, definitely opt for a reach weapon. That way BB procs when they move 5ft away, and then you lock them down with Sentinel at 10.)
The primary drawback is a lack of reliable advantage, so even though Psychic Blades add another layer of damage, missing on an opportunity attack risks them breaking Fear.
Thoughts on how to optimize around Fear+Sentinel, either by improving this build, or through some other means?
I would give some thought to taking Eloquence over Whispers: while the bonus damage is nice, the big thing is making sure you can reliably get Fear running, and Unsettling Words allows that reliably.
Your payoff here is permanently locking down one target for a whole combat as long as you can consistently land hits, and in that situation the damage rolls won’t really matter so much.
They don’t get another save, the rest of your party cleans up the others while you just plink away at your target (or just shoot at other targets, honestly, you don’t actually need to kill this one fast).
The primary drawback is a lack of reliable advantage,
For reliable advantage, the easiest approach would be Blindness/Deafness, but unfortunately I think that qualifies as the target not having line of sight to you anymore for Fear.
Shield Master won’t work either since the target will likely just stand from prone on their turn, before taking the AoO.
The Help action would give you a consistent source of advantage though (for only one attack, but that’s all you need), and it’s not too hard to get Find Familiar on a bard: using a Magical Secrets on it feels bad, but Ritual Caster or Magic Initiate would work. The normal risk of your owl getting swatted is much less since you can just move him into danger after your target is locked down from Fear.
It might not be worth the spell progression delay, but if you took Hexblade to 3 you could pick up Pact of the Chain for a familiar, which you could have be an imp for great flavor and a little extra survivability with invisibility.
Pact of the Tome also offers a familiar through the Book of Ancient Secrets invocation, but that doesn’t feel like it’s as worth the delay.
