Luckily for us, if my judgement is wrong for the offensive challenge rating and it should stay at CR 14 instead of 15, the final challenge rating is the same due to rounding.

So, 150 hit points brings us to CR 6 and the AC increases that to CR 7.

This creature isn't meant to be a solo monster, as evidenced by its aura ability, which means it is meant to be included as part of a group of enemies fighting a higher-tiered party of adventurers. Additionally, its Abjure Enemy only targets a single creature, not a whole party. Ultimately I would say that it doesn't affect the ranking.

However, we can see that frightening an enemy does have an effect! Having Frightful Presence, an ability that targets a large number of enemies, increases your effective hit points by a large amount if you're fighting a party of 10th level or lower.

The aspect of vengeance has 150 hit points, AC 18, and some saving throws, immunities, and flight. The saving throws and immunities are not sufficient enough to affect its ranking, and without ranged weapons to take advantage of its flight, it doesn't impact the ranking either.

It's still a potent ability. I'll give it the benefit of the doubt and say that it increases the attack bonus by another +1. This finally brings our offensive CR up by one ranking.

What about Abjure Enemy? It's unclear, because there aren't any precedents in the DMG about the impact of affecting an opponent's ability to escape a fight. However, the dire wolf and other creatures that can knock enemies prone (an ability which also halves an enemy's speed and makes them vulnerable) don't seem to gain any benefit from it in terms of CR.

87 damage is CR 14. Permanent advantage, as seen with Pack Tactics, grants a +1 to effective attack bonus. Even upping this effective bonus to +2 doesn't change the ranking in this situation, unfortunately.

The aspect of vengeance deals 29 damage with every strike of its maul, for a total of 87 per turn. Additionally, it has an ability which grants it, basically, permanent advantage.

Gloom Stalker

Gets bonuses on first turn of combat, like the scout. I'll give it the same kind of multiattack that only functions on turn 1, plus the bugbear's Surprise Attack trait that gives extra damage to creatures it has surprised.

Since it's inspired by drow tactics, we'll give it poison abilities similar to the drow. I like to use the illustration in Xanathar's to determine the weapon it uses but in this instance I'll do my own thing because the pickaxes / shortsword combo doesn't really make sense for the abilities it has; I'd prefer a ranged weapon. It'll use darts because it shouldn't be able to multiattack with a crossbow, and a shortbow doesn't ... feel ... right. And also because monster statblocks are a fun way to incorporate lesser-used weapons such as the dart anyway.

Umbral Sight can be simplified to Darkvision, but the "invisible in darkness" ability is cool enough to keep.

Iron Mind isn't crucial to the concept of the class. It's nice but it's not necessary, so I won't include it.

Same with the flurry ability which is also really finicky for the DM. Unnecessary.

The shadowy dodge ability is cool, simple, and thematic, so we'll add a Reactions section and include it there. We'll even boost it by removing a limitation -- the stipulation "as long as the attacker doesn't have advantage on the roll" will simply not be mentioned, because the flavour is what we're going for, not the balance.

Most regular ranger abilities don't contribute to the "gloom stalker" aesthetic except for Hide in Plain Sight and Vanish, which I'll include (in simplified form).

I'll take out spellcasting because it isn't critical and it adds a layer of complexity.

First turn, two attacks with poison darts (1d4 + 3 piercing, 2d4 poison, Con save or asleep like the drow poison). Ambusher gives extra 2d8 damage. Next two turns add two more attacks with poison darts. Divide by three for total: 17 damage per round.

That's CR 2. We'll give it a +5 bonus to attack to boost it to CR 3. Let's keep it at CR 3 -- we only need to get to CR 2 in defense, so we'll aim for that.

We'll lump all of its stealth abilities together -- a goblin's nimble escape gives it +4 to AC and ... attack bonus ... which actually boosts our offensive CR to 5!

I want to keep it at a +2 prof bonus, so CR 3 maximum for defense.

Shadowy Dodge itself adds a bonus to defense, which we'll simplify to +2 AC as a parry equivalent.

+6 effective AC. If we start at AC 13, we want to be at least three steps down from CR 3. We can give it leather armor and boost its actual AC to 14 without changing this.

Hit points would therefore be between 50-70 HP. I still think that's too much for a squishy ambusher so I'll make it between 36 and 49.

Constitution will be +1. 8d8 + 8 gives us 44 which is good. Let's squish it a bit down to 38 (7d8 + 7).

So, Defensive CR 2, 38 HP, 14 AC, +6 effective AC.

Offensive CR 5, 17 DPR, +5 attack bonus, +4 effective attack bonus.

Total CR: 7/2=3.5 round up to 4.

Challenge: 4 (1,100 XP).