Something I very much enjoyed in an encounter I ran yesterday, and that the players... well, reacted to appropriately, was giving an important rival NPC the ability to use Vicious Mockery as a Legendary Action. And it made a pretty immediate difference for me as far as playing her and giving the players some added incentive to engage with her.

Using Vicious Mockery at least once most rounds was a great mechanical reminder to include banter and taunting during combat, along with descriptions of attacks and spells going off. It was particularly fun using it immediately after a PC's turn to critique their technique or abilities, as well as adding some injury to the insult if they failed the saving throw. But even though my party made three of the four saving throws against it, the remarks themselves were enough to earn their ire and push an NPC they already hated up to LOATHE status... doubly so when she escaped. They've dealt with this character a couple times in non-combat situations earlier in the campaign, but this was the first time they've crossed swords with her, so it's really ratcheting up their dislike for her quickly.

It's a trick I'll be using on future Legendary NPCs I want the party to have a strong reaction to. Preparing some insults that are appropriate to the NPC helps figure out their voice and character, and having a mechanic to use them helps me make sure I don't forget to employ them. And doing so as a Legendary action means I can keep the regular action free for more mechanically impressive tricks. In the future, I may not even make it explicitly Vicious Mockery (taunting effects that work like Goading Attack, Mind Sliver or Menacing Attack could all work equally well, depending on the mechanical impact I want to be in play), but I'll definitely be including it in my toolbox.