It's posible to effective Eldrich Knight/Arcane Trikster MC?

[Erfar]

Look at thread's name

[JellyPooga]

Well it depends on your definition of "effective".

Any build with a level of Rogue has Expertise in (at least) 2 skills, making them "out of the ball-park" effective in those two skills. Cunning Action is also a very good ability, as is MHL.

Given that you're specifically asking about EK/AT, though, I presume you're most interested in the spellcasting...and that's where it really falls down. Not only are you only a 1/3 caster, you will probably be artficially slowing and/or reducing your potential due to rounding down two out of three levels. An EK 5/AT 5, for example, is effectively a 2nd level full-caster with 1st lvl spells.

Additionally, some of the EK and ATs best features don't come online until the level 7+ range. This means that for a 50/50 MC, you're not looking at getting them until near epic levels OR delaying low level abilities of the second class until a point when they have less impact (e.g. getting an extra 1d6 Sneak Attack is great at low levels, but doesn't mean so much at level 12 when HP totals are that much higher).

Then you have to consider that they both select spells from the same list. Not a big deal, perhaps, but when you consider that multiclassing Ranger or Paladin will not only give you more spells/day, faster, as well as giving you access to a wider variety of spells to choose from AND giving you most of what Fighter offers in terms of raw fighting ability, it's almost (not quite though) a no-brainer to favour a Rogue/Ranger or Rogue/Paladin over Rogue/Fighter, if you're looking for spellcasting as a focal point.

Having said that, EK/AT done carefully can give results. It's hard to make an actively bad character in 5ed and this is certainly not one of those ways. You get Extra Attack, Action Surge, Expertise and Sneak Attack as well as magical backup...what's not to like?

[Specter]

Why not? Any EK can benefit from at least 5 rogue levels (uncanny dodge, sneak attack, cunning action), and any AT can benefit from at least 8 EK levels (extra ASI, extra attack, war magic, action surge, fighting style). Though if I were doing it, I'd go the melee way and not the ranged one.

[Grod_The_Giant]

You'd be a pretty useless caster, at least at anything beyond cantrip level, but you'd be a pretty decent martial-- Fighter 5/Rogue 5, say, is a good build all by itself.

[tomato]

Why not? Any EK can benefit from at least 5 rogue levels (uncanny dodge, sneak attack, cunning action), and any AT can benefit from at least 8 EK levels (extra ASI, extra attack, war magic, action surge, fighting style). Though if I were doing it, I'd go the melee way and not the ranged one.

Well, first, think about the fact that compared to your level 13 character (8 EK/5AT) a normal EK would be getting three attacks on an action, and a level 13 AT rogue would be getting 7d6 sneak attack dice. Action Surge and an extra attack are well and good, but by this point, either one of these classes could be equipped with Haste. Additionally, once a rogue's sneak attack has been exhausted for the turn, extra attacks don't give them tons of benefit. They have smaller weapon dice, and don't usually get a chance to optimize multiple attacks the way that a full fighter would. Likewise, a full, level 13 EK fighter is not usually going to be using features like War Magic, and features like Fighting Styles or extra ASIs naturally lend towards multi-attacking characters due to how they're designed, with some exception.

[Citan]

Look at thread's name
Yes, it's very possible.
In fact, I would safely say it is usually a very nice idea, because Fighter and Rogue by themselves combine pretty well.

The hard choice will be on whether you want 3rd level spell or not.
If you bear being without it, then the best split is probably Fighter 11 / Rogue 9.

1. First approach: Fighter 11 / Rogue 9
You get 3 attacks per Attack, decent Sneak Attack, both Eldricht Strike and Magical Ambush so you have a good chance of inflicting spells, as many ASI as most classes, and a bunch of great defensive features (Indomitable, Uncany Dodge, Evasion) and other features (Expertise, Action Surge).
With a good starting DEX, CON and INT (High Elf? Human? Gnome?) it makes you very good overall. And you end with as many spell slots as a pure EK or AT (unless your DM is harsh enough to go with the most restrictive interpretation of PHB when you divide and round down each class separately).

BUT... You will only have 2nd level spells at most, so that may be a deal breaker to you. Or not. Mainly depends on if you see the spellcasting as "sporadic tricks" (such as Manoeuvers). If yes, this is a good build. If not, you will be frustrated by the too few resources.
On the plus side, you are a very solid martial (either War Magic + Sneak Attack bonus action, or TWF 3+1 attacks including Sneak Attack, every turn).

Another way to go at it would be to choose either Eldricht Strike OR Magical Ambush as your main spellcasting related feature, and build around it.

Second approach: unbalanced dualclass
2a. Eldricht Knight 13 / Rogue 7. you get 3rd level spells, Eldricht Strike, 3*attacks etc. From Rogue, Expertise, Uncanny Dodge, Evasion.
2b. Eldricht Knight 7 / Rogue 13. You go Fighter up to 5 for Extra Attack (unless you plan on using always Booming Blade and wield a shield), 6 for ASI or maybe even 7 for War Magic then don't look back.

Honestly though, my opinion is that for your goal, it is a hard choice because barring Constitution proficiency (1st level), armor proficiencies (1st level) and Action Surge (2nd level) which are great boons for minimal investment...
- Spellwise, it opens two other schools, which is nice, but the number of spell known and slots is weak compared to a dip in Wizard.
- Extra Attack is a nice alternative tactic when paired with Two-Weapon Fighting for cases where neither Booming Blade nor GFB are the best choices (if only because you just want an extra chance). But for a Rogue with Sneak Attack and Booming Blade, this is just a nice touch, not an essential.
- War Magic is actually a feature that shines a new bright in your specific case. On a normal Eldricht Knight, it's "meh". On your multiclass, it means you get the best of both: Booming Blade extra damage and rider, AND an extra chance of inflicting Sneak Attack, without even requiring two-weapon fighting.
So you essentially grind 3 levels just to reach War Magic, and it prevents you from getting "free-school" 3rd level spell, Blindsense, and Wisdom saving throw proficiency.

So for a Rogue chassis, it may be more worthwhile to actually stop Fighter leveling at 2-3 (if you really want the cool warding bond weapon feature), and instead go full Rogue to get the higher level features or dip into Wizard (Bladesinger is very fitting).
Third approach: triclass, Rogue main
3a. Fighter 2 / Arcane Trickster 15 / Bladesinger 3: Bladesong benefits, bunch of additional 2nd level spells, as good slots as a pure Trickster.
3b. Fighter 2 / Arcane Trickster 11 / Wizard (any) 7: you get up to Reliable Talent as a Rogue, you still get Magical Ambush, and you have so many spells up to 4th level at your disposal... ;)

Same in reverse: for a Fighter chassis, the best the Arcane Trickster brings is actual Rogue class features, Evasion and Uncanny Dodge. As far as spellcasting go, it's strictly better to dip into Wizard directly.
Fourth approach: triclass, Fighter main
4a. Fighter 13 / Bladesinger 2 / Arcane Trickster 5: you get mainly Expertise, Sneak Attack and Uncanny Dodge from Rogue, 3rd level spells.
4b. Fighter 11 / Bladesinger 2 / Arcane Trickster 7: you ditch 3rd level spells for the defensive Evasion.
Note that I don't feel taking less than 11 levels a good idea, because main schtick of Fighter is to hit, and only with Improved Extra Attack can the Eldricht Strike be worth it (and in general plain weapon attacks).


----------------------
To summarize...
1 (EK 11 / AT 9). Very balanced and versatile martial, overall very good, but only 2nd level spells max. Because of that, probably not the most "efficient".
2a (EK 13 / AT 7). You are an Eldricht Knight that sacrificed Haste and any 4th level spell to get a bit of extra damage per turn (Sneak Attack) and powerful defensive features. Two-weapon fighting recommended.
2b (EK 7 / AT 13). You are an Arcane Trickster that sacrificed Haste and any 4th level spell to get Action Surge, Fighting Style, an ASI and War Magic.
3a.(Fighter 2 / AT 15 / Bladesinger 3) You are an Arcane Trickster that sacrificed 4th level spells to get Action Surge, Fighting Style, much better AC and Concentration 2*short rest, many more spells (including rituals) and nearly as good slots as a pure Bladesinger.
3b. (Fighter 2 / AT 11 / Wizard 7): You are an Arcane Trickster that ditched everything above Magical Ambush and Reliable Talent to get Action Surge and great 4th level spellcasting, along with 2nd and 6th level School features (Bladesinger, Diviner, Abjurer, Necromancer are all good choices).
4a. (EK 13 / AT 5 / Bladesinger 2): you are an EK that trades 4th level spell and 4th attack to get Expertise, small Sneak Attack, Uncanny Dodge, and better AC+concentration twice per short rest.
4b. (EK 11 / AT 7 / Bladesinger 2): you sacrifice 3rd level slots to get Evasion (compared to previous).

All of these build will fare well in the end. Just choose depending on how you envision your character and maybe the rolls and race you pick. ;)

Personally, I'd go with either 4b to enhance the martial aspect, or 3b, so I get plenty of nice spells to use with Magical Ambush. Provided Bladesinger is available.
Otherwise, option 1, because while it is certainly not the most "optimized" (you get only 2nd level spells so it's a bit of a waste of Magical Ambush / Eldricth Strike) it's a very solid martial overall. ;) Although spellcasting wise, the progression would feel extra slow.

[Arkhios]

What Citan said, but here's my two copper pieces:

I tend to look at these issues from the "what would be cool" perspective more than from the "what would be super effective over 9000!", trying to find some middleground in optimal.

Considering that both the AT and EK are 1/3-casters, optimally I'd say take 6 or 9 levels each, and continue with Wizard until the end, depending on how much you wish to rely on spells and how much you wish to rely on your other assets (=class features).

AT 6/EK 6/Wizard 8 would end up with a total "caster level" of 12 (2+2+8), meaning you'd have up to 6th level spell slots at the highest, and up to 4th level Wizard Spells known (16+ first level spells - I'm not sure if there are that many to be honest - and roughly 4 spells per 2nd, 3rd, and 4th wizard spell levels)

In addition, you'd have expertise in 4 skills, sneak attack for 3d6 extra damage, one extra attack, action surge and a total of 5 ASI. (Plus other things)

Another way could be taking AT 9/EK 9/Wizard 2, ending up with a total "caster level" of 8 (3+3+2) with only up to 2nd level spells known, but likewise a lot more handy class features from both the AK and EK, and a handful of Wizard features.

[Citan]

What Citan said, but here's my two copper pieces:

I tend to look at these issues from the "what would be cool" perspective more than from the "what would be super effective over 9000!", trying to find some middleground in optimal.

Considering that both the AT and EK are 1/3-casters, optimally I'd say take 6 or 9 levels each, and continue with Wizard until the end, depending on how much you wish to rely on spells and how much you wish to rely on your other assets (=class features).

AT 6/EK 6/Wizard 8 would end up with a total "caster level" of 12 (2+2+8), meaning you'd have up to 6th level spell slots at the highest, and up to 4th level Wizard Spells known (16+ first level spells - I'm not sure if there are that many to be honest - and roughly 4 spells per 2nd, 3rd, and 4th wizard spell levels)

In addition, you'd have expertise in 4 skills, sneak attack for 3d6 extra damage, one extra attack, action surge and a total of 5 ASI. (Plus other things)

Another way could be taking AT 9/EK 9/Wizard 2, ending up with a total "caster level" of 8 (3+3+2) with only up to 2nd level spells known, but likewise a lot more handy class features from both the AK and EK, and a handful of Wizard features.

True that. You covered a case I didn't, where you just don't care about Extra Attack because you use only weapon cantrips, and don't care about Magical Ambush because it's not your style or you just don't want to play with save or suck spells. ;)

I'm still very partial to alter your AT 6/EK 6/Wizard 8 suggestion to substract one Wizard level to bring one Fighter level, because in my opinion this is one of the few cases when War Magic really brings much value. Basic uses:
- Booming Blade / GreenFlame Blade + weapon attack: 2 chances at inflicting Sneak Attack.
- Blade Ward + weapon attack: hopefully you hit, so no damage loss, but you get a nice protection against damage. Since Shield is expensive for the character, and Uncanny Dodge works only for one attack, this is a good way to reduce damage over time.
- Shocking Graps + ranged weapon attack: you want to go away from one enemy to hit another at range. With Shocking Grasp, you deprive him of reaction, before making your ranged attack at the one target you wanted to hit. Strictly better (unless I missed something) than using Disengage as a bonus action, moving and using your Attack action. Because with this method you dealt a bit of damage and your allies can also move freely around this one.
So I think it's worth 2 less caster levels, but obviously this is a matter of personal taste. ;)

And the reason why I didn't suggest the AT 9 / EK 9 / Wizard 2 is mainly because I didn't see much value in 8th and 9th level of Fighter (compared to levels in caster), and in broadened view I don't think War Magic is worth the whole investment, unless you really wanted also the extra ASI at 6th level. In which case this is indeed the best way to go.

[Grod_The_Giant]

I'm still very partial to alter your AT 6/EK 6/Wizard 8 suggestion to substract one Wizard level to bring one Fighter level, because in my opinion this is one of the few cases when War Magic really brings much value. Basic uses:
I think I'd tweak it just a little more and shave off a Rogue level to replace the Wizard-- the extra slot and ASI are probably better than Evasion. Bringing you to EK 7/Rogue 5/Wizard 8. Bladesinger might be fun.

[Specter]

Well, first, think about the fact that compared to your level 13 character (8 EK/5AT) a normal EK would be getting three attacks on an action, and a level 13 AT rogue would be getting 7d6 sneak attack dice. Action Surge and an extra attack are well and good, but by this point, either one of these classes could be equipped with Haste. Additionally, once a rogue's sneak attack has been exhausted for the turn, extra attacks don't give them tons of benefit. They have smaller weapon dice, and don't usually get a chance to optimize multiple attacks the way that a full fighter would. Likewise, a full, level 13 EK fighter is not usually going to be using features like War Magic, and features like Fighting Styles or extra ASIs naturally lend towards multi-attacking characters due to how they're designed, with some exception.

Haste costs an action, and if it gets dispelled or if you lose your concentration you're screwed. It's not as cool as everyone seems to think it is, and this I say with a scarred heart. Extra Attack is great for rogues not for average damage, but because there's always a 1 or 2 or 3 creeping up on the first die, and losing all your damage on a turn is too sad.

[djreynolds]

Every fighter/Rogue I make is basically strength based shield master expertise in athletics with a 13 in Dex. EK/AT just adds spells to it.
Use uncanny dodge when your shield spell runs out. Mirror image blur pro from evil

Or flip it and go the dex route. Lots of misty step, invisibility, shield spell and BB GFB.

You don't need attack spells, your SA and BB will cover damage. You need mobility, protection, etc.

[Citan]

I think I'd tweak it just a little more and shave off a Rogue level to replace the Wizard-- the extra slot and ASI are probably better than Evasion. Bringing you to EK 7/Rogue 5/Wizard 8. Bladesinger might be fun.
Very much possible. I guess it depends for a good part on your stat rolls and how often you're "experiencing" effects targeting DEX saves.
But since you get an ASI instead, you could very well just pick a feat that helps in that regard if you really want to get similar defense (thinking Shield Master here, since you have shield proficiency anyways. Or Resilient: Dexterity, which is arguably better if you started Fighter because you will fail much less saves in the first place ;)).

So, yes, in many cases, your suggestion probably holds more potential value. ;)

[Zene]

Haste is amazing for rogues. Use your haste action to attack. If you missed, use your regular action to try again. If you didn't miss, use your regular action to ready an attack, and potentially do your sneak attack damage twice in one round.

(In my experience)

[djreynolds]

Honestly, if you multiclass EK/AT, you may never see haste until later levels... you may have the spell slots to cast haste but lack the haste spell, 13th level of EK or AT for access

If you want haste earlier, the easiest route is to grab 5 levels of wizard for quicker access.

I think it is best to go EK or AT and then wizard if it is spells you want.

But EK or AT by themselves is just fine.

[Citan]

Haste is amazing for rogues. Use your haste action to attack. If you missed, use your regular action to try again. If you didn't miss, use your regular action to ready an attack, and potentially do your sneak attack damage twice in one round.

(In my experience)
Except that you don't strictly need Haste on an EK / AT multiclass, precisely because you can either go Extra Attack + bonus action (twf), or use Booming Blade + bonus action attack (War Magic).
So you always get at least 2 chances to land Sneak Attack whatever happens.

With that said, Haste value is even better on a multiclass than on a pure Rogue, in the sense that you get better Concentration saves so less chance to end in a tough spot. And double mobility frees the bonus action for weapon attack instead of Dodge.
So while not "necessary", it's still a very great option indeed. ;)