MY Homebrew twist of the Arcane Trickster to Arcane Assassin: Version 1. (Currently trying it out for balance issues) ;)

Some rogues enhance their fine-honed skills of stealth and agility with magic to strike at their foes harder then before, learning ways of stalking and killing their prey in new ways. Most of these rogues get trained by an arcane master or a great house of magic, those trained at a house of magic usually ends up becoming a spy/assassin or spy master of that great house and those trained under an arcane master can become that masters left hand, stalking the shadows and serving your master.

Spellcasting

When you reach 3rd level, you gain the ability to cast spells. See Spells Rules for the general rules of spellcasting and the Spells Listing for the wizard spell lists.

Cantrips

You learn three cantrips of your choice from the wizard spell list. You learn another wizard cantrip of your choice at 10th level.

Spell Slots

The Arcane Assassin Spellcasting table shows how many spell slots you have to cast your wizard spells of 1st level and higher. To cast one of these spells, you must expend a slot of the spell’s level or higher. You regain all expended spell slots when you finish a long rest.

For example, if you know the 1st-level spell charm person and have a 1st-level and a 2nd-level spell slot available, you can cast charm person using either slot.

Spells Known of 1st-Level and Higher

You know three 1st-level wizard spells of your choice. One of these spells must be Hunter's Mark or Hex which you gain as additional spells for this subclass.

The Spells Known column of the Arcane Assassin Spellcasting table shows when you learn more wizard spells of 1st level or higher. Each of these spells must be of a level for which you have spell slots. For instance, when you reach 7th level in this class, you can learn one new spell of 1st or 2nd level.

Whenever you gain a level in this class, you can replace one of the wizard spells you know with another spell of your choice from the wizard spell list. The new spell must be of a level for which you have spell slots.

Spellcasting Ability

Intelligence is your spellcasting ability for your wizard spells, since you learn your spells through dedicated study and memorization. You use your Intelligence whenever a spell refers to your spellcasting ability. In addition, you use your Intelligence modifier when setting the saving throw DC for a wizard spell you cast and when making an attack roll with one.

Spell save DC = 8 + your proficiency bonus + your Intelligence modifier

Spell attack modifier = your proficiency bonus + your Intelligence modifier

Arcane Assassin Spellcasting

Rogue Level	Cantrips Known	Spells Known	— Spell Slots per Spell Level —
			1st	2nd	3rd	4th
3rd	3	3	2	—	—	—
4th	3	4	3	—	—	—
5th	3	4	3	—	—	—
6th	3	4	3	—	—	—
7th	3	5	4	2	—	—
8th	3	6	4	2	—	—
9th	3	6	4	2	—	—
10th	4	7	4	3	—	—
11th	4	8	4	3	—	—
12th	4	8	4	3	—	—
13th	4	9	4	3	2	—
14th	4	10	4	3	2	—
15th	4	10	4	3	2	—
16th	4	11	4	3	3	—
17th	4	11	4	3	3	—
18th	4	11	4	3	3	—
19th	4	12	4	3	3	1
20th	4	13	4	3	3	1

Arcane Hunter

At 3rd level, when attacking a creature that's under the effect of your Hunter's Mark or Hex spell and you hit with your attack, you can make another attack with your bonus action to strike at the creature. (possibly hitting your mark twice in one round).

Also at 3rd level, you've learned to think one step ahead of your enemies, be on your toes and use that intelligent brain of yours to kill before being seen. You can give yourself a bonus to your initiative rolls equal to your Intelligence modifier.

Magical Ambush

Starting at 9th level, if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn.

Arcane Shadow

At 13th level, you gain the ability to become invisible for 1 minute as a bonus action after you cast a spell of 1st level or higher. You become invisible like you cast the spell Invisibility this does not count as if you are concentrating if you already cast a spell that requires concentration.

You can also silently cast any spell if you are currently hidden, this means you do not reveal your position to any creature when casting a spell and make no noise casting. Or make a successful sleight of hand check if you aren't hidden before casting the spell. (DMs discretion)

Spell Assassin

At 17th level, when one or more creatures are under the effect of a friendly spell cast by you or your companions you can't miss your target(s). You get to roll with advantage on all your melee and range attacks against the affected target(s). Also rolling a 19 on a attack roll counts as a critical hit for you. This no longer applies if a creature is no longer affected by a spell.