3d6

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?