Need Help Building an Assassin

[LordAvalen]

Basically just the title. I want an in-and-out kind of assassin, who can kill quickly and quietly while deal big surprise damage. Bonuses are shadows and the ability to change faces. I'm kind of new to 5e and not sure what I should multi class. Home-brews are also welcome as I have a lenient DM (Just make sure to check it's at least a tad balanced). From what I can deal however, it's between the Assassin Rogue and Shadow Monk.

TLDR: Assassin build with high single target damage and good get aways / sneaking.

EDIT: Think "Maiden of Death" as concept.

[RickAllison]

I strongly urge you to consider a dip into Druid, especially up to 3. That gives you the ultimate infiltration mode (be any tiny beast you wish. Be a literal fly on the wall!), sustenance in the form of Goodberry for long times of hiding, Mending to remove evidence of your infiltration, Pass Without Trace for +10 to Stealth and to cover your tracks, Heat Metal to assassinate anyone wearing metal armor (at least up to an average 90 hp), and other benefits from your archetype. A dramatic rise in power for a relatively small dip.

[LordAvalen]

I strongly urge you to consider a dip into Druid, especially up to 3. That gives you the ultimate infiltration mode (be any tiny beast you wish. Be a literal fly on the wall!), sustenance in the form of Goodberry for long times of hiding, Mending to remove evidence of your infiltration, Pass Without Trace for +10 to Stealth and to cover your tracks, Heat Metal to assassinate anyone wearing metal armor (at least up to an average 90 hp), and other benefits from your archetype. A dramatic rise in power for a relatively small dip.

Do you know if there is any magic item that grants Pass Without Trace? I already have mending with the race I am using and the others aren't as critical for the class.

[Osrogue]

Warlocks make excellent deceivers and can take an ability that allows them to change their face at will, and can cast some illusions at will. Some warlocks can make summon a magic weapon to leave no murder weapon behind. Combine with rogue (take expertise deception and stealth) for the sneak attack damage. Warlocks make excellent social assassins, with a bit of illusionist flair.

Shadow monks can use pass without trace, an assassin's best friend, and after six levels, teleport through shadows and stun targets. (Take expertise stealth and perception) These make good kill you in your sleep assassins.

Trickery clerics have some neat tricks for assassins like disguise self, dimension door, poison damage on a strike, pass without trace, and at will advantage on stealth, along with a cool illusory double ability. Take for divine assassin. Mix with rogue.

A straight X levels or rogue is really all you need for a vanilla assassin though.

[Lindonius]

The true munchkin assassin is this one.....

https://www.reddit.com/r/DnD/comments/3rdzu7/5e_rogue_assassin_nova_build_questions_too_good/

Looks good on paper but has a few disadvantages, namely no initiative boosting abilities and burning through all resources in like 1 round. Good luck if there's anything other than your mark to have to deal with afterwards.

[Specter]

Big sneak means Expertise in Stealth. Big damage means Assassinate and Extra Attack.

If you want all of those, go Rogue 1, then take 5 levels in Ranger, then proceed as Rogue. Pass Without Trace is now your middle name. Druid is another way to go, but no Extra Attack makes your surprise rounds meh.

Also, Alert is a mandatory feat.

[Corran]

Some levels in rogue (assassin), in order to get assassinate (level 3), are mandatory for what you want to achieve.

Since you want to use disguises a lot, I would suggest to dip 3 levels in warlock, so that you take the invocation mask of many faces (at warlock 2) for at-will disguise self essentially. You would have to take the actor feat in order to make diguise self work as you intended though (to change your voice, among other good benefits the feat provides), so that dip comes hand in hand with a feat investment. Besides that, you get some really useful cantrips (thinking mainly of booming blade), some really useful spells (hex which lowers initiative, invisibility, darkness), and if you go blade pact, you gain the ability to sneak one weapon anywhere effortlessly. Plus some other minor benefits. I would sugest human, in order to grab the actor feat asap, and you can take devil's sight as your other invocation (at warlock 2) to deal with the lack of darkvision that vhuman suffers from. And besides, this way you can put to work the darkness + devil's sight + cunning action hide + hit with booming blade and move away without provoking OAs due to darkness. And the classic BB + cunning action disengage.

If you dont want to invest in the actor feat, then you are looking at alter self. In this case, I would recommend a sorcerer progression (http://www.giantitp.com/forums/showthread.php?475714-Need-help-with-the-spell-selection-for-my-heavy-sorcerer-cutthroat). Playtested up to level 11 and it works well.

Or, you can just select a doppelganger (edit: changeling) as your race, boom, easy. And perhaps play a singleclass rogue assassin who relies on sneak attack for his damage. Or multiclass however else you like.

A note about multiclass assassin builds. Most such builds that I've seen here and there in the internet, focus too muh on assassination damage, at the cost of efficiency. Meaning that most such builds end up dealing some incredible damage when/if they manage to get a surprise attack, but are very underpowered in everything else they do, and quite often there is some serious lack of synergy in them too. Be careful not to fall in that trap, as such builds dont deal too well when playing as part of a group, mostly because you dont get to enjoy it yourself.

Anyway, let me leave you with a few thoughts in general. Regarding feats, alert and lucky and pretty much mandatory. Take them asap. Try to grab invisibility from somewhere, the ability to be unseen is too useful to an assassin. Expertise stealth, obviously, and I would suggest deception too, judging from what you want to accomplish (deception rolls come a lot into play when you are using disguises).

And a question. What weapon do you see this character wielding?

[Belac93]

Lets see, perfect assassin? What about changeling ranger 5/assassin x? x2 prof bonus +15 is nothing to sneeze at.

Although, ranger 10 rogue 10 would be pretty awesome. At level 20, with hide in plain sight, that is +37 to stealth.

By the way, anyone with rogue 11 and expertise in stealth can never get lower than a 23 on stealth.

[Specter]

Lets see, perfect assassin? What about changeling ranger 5/assassin x? x2 prof bonus +15 is nothing to sneeze at.

Had a player like that in a campaign of mine. When the party was in need of money, he created an elaborate ruse to kidnap himself and ask for a fat ransom. Good times.