Start with 5 levels of gloom stocker ranger. Getting favored foe, umbral sight, ambush abilities and extra attack.
Then I take 6 levels of Shadow Sorcerer. Getting extended dark vision 150 feet when combined with my Gloom Stocker abilities. I'd also get Hound of Ill Omen, a controllable ally. The spells would largely be used to buff myself or others, including mage armor. My sorcerery points would be used to extend spells or allow me to cast them subtly. I'd also take Shadow Blade which I'd be able to upcast to 4th level.
Finally, 9 levels of Soul Knife Rogue. Getting 5 d6 sneak attack as well as a bunch of expertise. The combination of Shadow blades and my soul blades allow this character to walk around with out weapons. Mage armor allows me to walk around in civilian clothing.
This build gives me many ways to generate my own opportunities for sneak attack between Hound of Ill Omen, being invisible in the dark and shadow sorcerers dark vision abilities.
Like Arn has alluded to I think Warlock is probably the best route for you to take with this and could multi into Rogue easily enough without such a wide spread of stats. I would recommend Hexblade as it will grant extra bonuses to your weapon. If you desperately want Hunters Mark as well, you can pick up Feytouched (CHA) feat from Tashas and pick it up that way. You'll have access to some summoning spells as well which can operate fairly similar to the role of your Hound of Ill Omen in this instance.
I should have posted my stats but I'm not going heavily SAD. I'd ideally have a 20 in dex and nothing else higher than 14 (8 str, 20 dex, 14 con, 9 int, 13 Wis and 14 Cha). The reason for this is that the spells I'll be casting largely won't rely on my spellcasting modifier.
Planning out a level 20 character is rarely a good idea unless you are going to start out in Tier 4 to begin with. Few games ever last long enough to get there otherwise. Tier 4 is honestly the most difficult for a DM, as the rules are breaking down in various ways and it is hard to challenge players anymore. It can be done, but only by people with a lot of experience in that, and there you are again, you need experience to get experience, and few have that at Tier 4 since so few play at that level.
I've seen builds that were way, way more powerful. You didn't mention Feats, or Magic Items. A lot will depend on the setting and the other players, if there are any.
The Ranger levels seem fine.
Your Shadow Sorcerer's Hound of Ill Omen isn't going to last long. You're 11th level now. Three Sorcery Points, it lasts for 5 minutes, but can only attack one target. It can't change targets, so that's not all that handy. It's a Challenge Rating 1 Monstrosity with a 14 AC, and an average of 40 hp. It might easily be killed quickly at that level of play. It only gets to use the Help action or Bite, and while it gets Advantage on the hit roll for that bite if you are within five feet of it's target, the damage isn't anything to write home about.
The Shadow Blade levels seem all right, but you only get those Soul Blades at level 20 so I'm not sure that's going to be much use.
Going with Hill Dwarf as my race.
Start with 5 levels of gloom stocker ranger. Getting favored foe, umbral sight, ambush abilities and extra attack.
Then I take 6 levels of Shadow Sorcerer. Getting extended dark vision 150 feet when combined with my Gloom Stocker abilities. I'd also get Hound of Ill Omen, a controllable ally. The spells would largely be used to buff myself or others, including mage armor. My sorcerery points would be used to extend spells or allow me to cast them subtly. I'd also take Shadow Blade which I'd be able to upcast to 4th level.
Finally, 9 levels of Soul Knife Rogue. Getting 5 d6 sneak attack as well as a bunch of expertise. The combination of Shadow blades and my soul blades allow this character to walk around with out weapons. Mage armor allows me to walk around in civilian clothing.
This build gives me many ways to generate my own opportunities for sneak attack between Hound of Ill Omen, being invisible in the dark and shadow sorcerers dark vision abilities.
Like Arn has alluded to I think Warlock is probably the best route for you to take with this and could multi into Rogue easily enough without such a wide spread of stats. I would recommend Hexblade as it will grant extra bonuses to your weapon. If you desperately want Hunters Mark as well, you can pick up Feytouched (CHA) feat from Tashas and pick it up that way. You'll have access to some summoning spells as well which can operate fairly similar to the role of your Hound of Ill Omen in this instance.
I should have posted my stats but I'm not going heavily SAD. I'd ideally have a 20 in dex and nothing else higher than 14 (8 str, 20 dex, 14 con, 9 int, 13 Wis and 14 Cha). The reason for this is that the spells I'll be casting largely won't rely on my spellcasting modifier.
Planning out a level 20 character is rarely a good idea unless you are going to start out in Tier 4 to begin with. Few games ever last long enough to get there otherwise. Tier 4 is honestly the most difficult for a DM, as the rules are breaking down in various ways and it is hard to challenge players anymore. It can be done, but only by people with a lot of experience in that, and there you are again, you need experience to get experience, and few have that at Tier 4 since so few play at that level.
I've seen builds that were way, way more powerful. You didn't mention Feats, or Magic Items. A lot will depend on the setting and the other players, if there are any.
The Ranger levels seem fine.
Your Shadow Sorcerer's Hound of Ill Omen isn't going to last long. You're 11th level now. Three Sorcery Points, it lasts for 5 minutes, but can only attack one target. It can't change targets, so that's not all that handy. It's a Challenge Rating 1 Monstrosity with a 14 AC, and an average of 40 hp. It might easily be killed quickly at that level of play. It only gets to use the Help action or Bite, and while it gets Advantage on the hit roll for that bite if you are within five feet of it's target, the damage isn't anything to write home about.
The Shadow Blade levels seem all right, but you only get those Soul Blades at level 20 so I'm not sure that's going to be much use.
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