So, I'll be the first to admit that even though I create custom monsters on a fairly regular basis, this is one of those things I hope they revise for the 2024 edition, because the instructions in the DMG are a bit of a mess. I mean, if you went by the DMG - AC 18, HP 281-295, Attack Bonus +8, DMG/Round 93-98, Save DC 18. None of the monsters in the published books seem to match that.
So if I look at some of the more recent monsters, say Umbraxakar a huge dragon from Dungeon of the Mad Mage we have AC 19 (a bit higher), HP 212, while in dim light has resistance to damage that isn't force, psychic or radiant, good saves, etc. He has 3 attacks, a bite at +12 to hit for 18 dmg and 2 claws +12 to hit, 14 damage each. So if all 3 hit 46 points of damage in a single round.
But he also has a breath weapon that does 66 points of damage, probably to multiple PCs which really ups his average. He also has some fun things like a repulsive breath to push people around and can fly away without taking opportunity attacks.
Or take Typhon from Mythic Odysseys of Theros. His AC is 17, and has 195 HP but also regenerates 20 HP every round unless it takes radiant. He's +12 to hit, with 3 attacks, bites for 35 damage, a constrict that grapples and does 17 damage and 17 ongoing damage until the target escapes (up to 2 creatures), a maw that does 26 piercing plus 19 acid damage. So total of 78 damage in one round with up to 34 ongoing damage between two creatures. Typhon is not legendary so no legendary resistance or actions.
So we have 2 very different monsters, neither of which match up to the DMG. The dragon Umbraxar has a decent armor class, probably opens with a breath weapon, does it's best to split up the party to divide and conquer. Typhon is a brute, wade into combat and do a bunch of damage while regenerating unless it takes radiant damage.
Meanwhile your guy is just going to wade into combat, likely invite attacks and does 34 damage if they hit. All while likely being reckless to invite even more damage. So ... yeah. He needs to be beefed up significantly and given some cool actions.
Breaking it down a bit, I'm perfectly okay with a demonic brute but if you want this to be a legendary they should do cool stuff.
AC/HP - if you're going to make this a brute, leaving the AC at 17 is okay, but it also kind of depends on your group. You know them better than I do but many groups are going to chop him into pieces quickly. I'd up the HP but also give him 20 HP regen per round.
Speed and movement - do your PCs fly? If they do I'd add a fly speed and big leathery bat wings. Ability scores, etc are all fine. One of the things I ignore for higher level PCs is the resistance to damage adding to the CR of the monster, it doesn't really come into play. Fear aura is fine. I probably wouldn't bother with reckless.
Next is the legendary resistance. This is a feature, not a legendary action and he should have 3 (at least). I actually make it the number of players -1, but I'm a mean DM.
Legendary Actions, again I make this number of players -1. No need to take it easy on the PCs.
The attacks are okay, but should do significantly more damage. Let's say 17 slashing damage plus 17 (5d6) fire or cold damage. Make a dex save DC 19 or take 18 ongoing fire or cold damage, the Aspect's choice.
I'd also give them a recharge ability, perhaps he attacks everyone within 10 feet while moving his base speed (no opportunity attacks), attacking with advantage. If he hits do normal hit damage and make a strength save DC 19 or be thrown 20 feet away.
Legendary Actions?
- Give him a fly speed and the copy the dragon's fly away attacking and knocking people prone bit for 2 actions or just let him move away without provoking. You don't want him surrounded too easily.
- Legendary action attack is fine.
Anyway, those are my thoughts. I have a tendency to make these kind of monsters really tough, but they kind of need to be. Especially if they don't always have lair actions to back them up.