Would letting the Assassin rogue replace its 9th-level Infiltration Expertise feature with the Thief's Supreme Sneak feature be unbalanced?

Question

I wanted to ask about allowing the Rogue sub-class (Assassin) to benefit from Supreme Sneak.

I have a player in my campaign who plays as an Assassin Rogue and would like to have access to the Supreme Sneak upgrade at level 9, which normally is part of the Thief build.

Her argument is that Supreme Sneak is more appropriate to an Assassin because of its synergy with Assassinate. I can see her point and I don't have any problems with allowing this, but I wanted to know if there are any glaring problems with doing this.

Would allowing a Rogue (Assassin) to benefit from Supreme Attack upon reaching level 9, instead of the regular Infiltrator Expertise, make the sub-class very unbalanced?

Here are descriptions of the relevant class features:

Assassinate (Assassin)

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.

Supreme Sneak (Thief)

Starting at 9th level, you have advantage on a Dexterity (Stealth) check if you move no more than half your speed on the same turn.

Infiltrator Expertise (Assassin)

Starting at 9th level, you can unfailingly create false identities for yourself. You must spend seven days and 25 gp to establish the history, profession, and affiliations for an identity

(PHB p.97)

Answer 1

I think this change would be fine

A common complaint about the Assassin roguish archetype is that its level 9 and 13 features are quite lackluster. They only make sense in specific scenarios, which may not come up in your campaign, making them completely useless. I agree with your player that Supreme Sneak does suit the Assassin from a flavour perspective, and if it helps that player to feel like their choice at level 3 wasn't a mistake, I don't see a problem with it.

Mechanically, it is a straight up trade. They aren't gaining Supreme Sneak in addition to Infiltrator Expertise, it's instead of. It also doesn't have any multiclassing issues, such as giving a feature from a different class to this character; any character would need 9 levels in rogue either way. This isn't a particularly strong feature, like the features that roguish archetypes gain at level 3; their level 9 feature is usually somewhat of a ribbon feature. Rogues generally are also considered to have a strong "core class", so they don't gain that much additional power from their archetype features.

I also don't see there being a balance issue in combining Assassinate with Supreme Sneak. Although I agree with your player that Supreme Sneak suits an assassin, I don't think it leads to any game-breaking combinations, since an Assassin rogue is very likely already extremely sneaky (proficiency and likely Expertise in Stealth; credit to Cubic for pointing this out first, but I wanted to back up my answer with this point also).

The only thing I would add is that it might be worth making sure your other players are alright with this change, especially since I imagine your Assassin player is the only one who will be getting this change. It shouldn't become a free-for-all where people can basically just mix-and-match their favourite features from all of their class's archetypes. Assassin is somewhat of a special case because of how weak their level 9 and 13 features are considered to be, given that they are very campaign-specific.

Answer 2

No, this change wouldn't be unbalanced

It actually wouldn’t make much of a difference at all. By level 9 either getting advantage/huge bonuses on sneak attempts or disguising yourself as another person aren’t terribly hard to pull off if you have spellcaster friends or access to magic items (Invisibility and Pass Without Trace just to name a few, let alone that your sneaky assassin probably has Expertise in Stealth anyway). Neither of these features are particularly strong to begin with, and swapping one for the other is neither going to make or break any particular build.

At any rate, Assassinate is going to remain hard to pull off reliably not necessarily because ambushing a target is hard (although depending on the exact circumstance it often will be), but rather because it’s entirely reliant on the Assassin rolling a higher initiative than their intended target, which neither feature helps with.

If it makes your player happy, I don’t see any issue with letting them have this.

Answer 3

It's only a bit unbalanced

As @NathanS comments, Assassin has weak Level 9 (Infiltration) and Level 13 (Imposter) features. You may not ever have a chance to use them. This is balanced by the powerful Levels 3 (Assassinate) and situational 17 (Death Strike) features. Compare this to the Level 3 Thief features (Fast Hands and Second Story Work), which give no advantage in combat and, again, may not ever be useful (I was in a game last night where the party noticed 'Hey, this might be the only case we've ever seen where Second Story Work could be useful!').

But it won't be needed very often

The thing is, by Level 9, if you have expertise in Stealth, any Rogue will likely have +12/13 (2 times +4 proficiency, plus +4 or +5 Dex modifier) to Stealth. By Level 11 your minimum Stealth roll becomes 22/23 from your Reliable Talent. Most enemies will never see you coming. However, when sneaking up on high level enemies like Dragons with high perception bonuses, this would be a big buff.

Personally, I'm not a fan of letting players combine features from different sub-classes because cafeteria-style feature selection takes away from the inherent give-and-take between choosing your archetype. But in this case, the times it will actually be useful might only come up a handful of times in a 20 level character progression, so I think it would be fine.

Answer 4

I would rule no. The give and take of Thief and Assassin is that Assassins get the very powerful Assassinate at 3rd level, but their 9th level ability is inferior. It's a trade-off, and I think it works best that way. Synergy is not a valid reason to change the rules, and in fact I say that it's a good reason not to, because it eliminates the drawback of not taking the Thief subclass. If an assassin wants advantage on stealth, they can just shell out the cash for some Boots of Elvenkind. If they can't get them, whether due to setting limitations or inability to raise money, tough luck.