A creature like one of the Laesuri isn’t some mook you encounter with a bunch of minions. She’s a solo that is darn hard to take down, so how do you approach that design? Well, you first need to know what level she will be encountered at. In this case, it’s level 13. Now you need to know if the characters will be fresh when they meet her. For this design we will assume the characters have had a full day before they get to her, so they aren’t completely fresh. You also need to know how big the character party is; for this setting, we are going with five. So, our monster will be designed to be a hard encounter for a party of five 13th-level characters that are at about 50–75% strength. That puts our monster at challenge rating 17 with legendary actions and saves since she’s a solo.
Now for abilities. What kind of damage is our creature going to do? We know we want her to have a big scream, and since she’s a harpy she should have some ability to charm with a song. She needs melee attacks, so we’ll give her big claws as well. Lastly, I wanted something that showed her supernatural lineage, so I also gave her sorcerer powers.
Whenever I design a monster I try to envision the fight in my head and think about all the things a party will do. We know the monster will be targeted by spells, and the monster’s spells will be countered in turn by the wizard. How do we account for that? We also know at least two melee characters will get into her face and try to one-shot her with Great Weapon Fighting and sneak attack. How do we account for that? Lastly, we can assume one archer will be on the edge of the map with the sharpshooter feat, so how do we account for that? Then, while you are accounting for all the optimizers, you also have to account for the non-optimizers. Here’s what I do.
I try to give the monster the ability to counter most of the mentioned scenarios and then put in the tactics cautionary guidelines for less optimized parties. At tier 3 the characters will have all kinds of shenanigans like invisibility, dimension door, globe of invulnerability, and other spells and feats designed to quickly ruin your monster’s day. To counter these, truesight should buy a round of defense against the rogue, counterspell for those wizards that forget to get out of range before casting and teleport to get out of the way of the big brutes. I also made sure all of these abilities are available as legendary actions. That’s the defense.
Offense is pretty straightforward. As a sorceress, she can cast a leveled spell as a bonus action and then attack with melee or a cantrip. That’s a lot of potential damage. I also make her able to use her legendary actions to cast spells as well. Now she’s really draining some hit points. Let's top it off with a big bang that makes players say “Damn! She was hard to beat!” How about a stunning scream? We’ll also put the encounter in the sky, so her stunning scream has falling damage on top of it. Oohhh Yeaahhh!
So now I have a stat block and the monster’s look; however, it still needs to be playtested. That comes later. Playtesting is essential when creating monsters and adventures—you will always make adjustments and catch things you didn’t see before. It's also always good to get another set of eyes on your creation if for nothing else than a different opinion.
And there you have it: Talanara, the Great Screamer.