I don't believe that a Rogue can normally sneak attack when it grapples. This is why (let me know if I'm wrong):
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Under Sneak Attack in the Rogue section it states
Once per turn you can deal an extra xd6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
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In Chapter 9 under Grappling it states
can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them.
So this to me is a special attack not a finesse or ranged weapon attack since it requires Athletics(Strength) to engage, and one free hand not holding a weapon, therefore Sneak Attack is NOT allowed imo)
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You would also need the feat Grappler from Chapter 6 which states
You have advantage on attack rolls against a creature you are grappling […]
So you apparently don't have advantage on attacks without this feat, so you can't use Sneak Attack.
So my conclusion is that a Rogue can only use sneak attack in a grapple if
- it has the Grappler feat, as well as
- is multi-classed with something like fighter/ranger/monk/etc. where at level 5 or so it gets a second attack, since grapple is a special (strength based) attack.
I am curious whether this works. I would love to make a strength-based monk 6 / rogue 1 to focus on a one-two grappling-then-assassinating tactic: one attack is to grapple and the second is to sneak attack. This would be a great way to get Sneak attack every round. Please correct me if I'm wrong in my understanding.
One last thing. If two creatures are in a grapple you just make the check at the beginning of the next turn so the lvl6 Monk/lvl1 Rogue could have two attack (one of course being sneak attack)? Or would you have to grapple attack again, and get one attack (finesse, w/sneak attack)?
Best Answer
You seem to have missed a critical part of sneak attack, and you seem to not understand the mechanics of grappling in much detail.
I'll deal with Sneak Attack first. I DM for a group that includes a rogue. He has been able to sneak attack about 95 % of his turns in combat so far, for one simple reason: the group has a tank that stands next to the enemies. The important part of Sneak Attack is as follows:
If your group has a tank, they are most likely going to stand next to the enemies. Just make sure you're attacking the enemies next to your tank, and you sneak attacks are going to be available pretty much every turn.
Now, on to grappling.
Grappling can never activate sneak attack, since they are special melee attacks, not special melee weapon attacks. Grappling replaces one of your normal attacks when you initiate the grapple. Once you have grappled the enemy, you don't have to commit anything on your turn to keep the grapple going. There are two ways a grapple can end: If you intentionally let go of the target, or if they try to break free. The first option requires that you actually say that you let go of the enemy. The second option forces you to do an opposed athletics check, but on their turn, using their action. Once you have grappled your target, you can use your action on subsequent turns to attack them. If you have the grappler feat, or if an ally is within 5 feet of the enemy (likely the tank of your group, if you do a bit of team coordination), you will be able to sneak attack them.
Your build seems to be trying to solve a problem that, for the most part, doesn't exist if you play in a group. Simply coordinating with your tank so you're attacking enemies within 5 feet of them will activate your sneak attack every time, unless you have disadvantage. Rogues in 5e are balanced around being able to sneak attack pretty much every turn. If they can't, they fall behind on damage compared to even the most tank-focused of fighters.