Surprise, Assassinate, Dual Wield, Great Weapon Master loop and Skulker

Question

I'm trying to find out if a particular surprise attack & assassination loop would work out:

1. It is the rogue turn. He surprise attacks with 2 daggers at the same time. Does he get a surprise attack with both?

2. Assuming the rogue got a surprise attack with both daggers, do both trigger assassinate?

   Assassinate

   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.

3. In case both trigger assassinate, would that be 2 critical hits? Would both critical hits each trigger Great Weapon Master?

   Great Weapon Master

   You've learned to put the weight of a weapon to your advantage, letting its momentum empower your strikes. You gain the following benefits:

   • On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.
   • Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add 10 to the attack's damage."

4. At this moment, would the NPC still be surprised? If so, could both attacks earned from Great Weapon Master trigger Assassinate? If yes, can I loop like this?

5. If not, can this skulker rogue do the following: in a lightly obscured place, try to hide in plain sight. If the mobs are still considered surprised this turn (are they?), and the rogue has actions left, could he assassinate again and return to step 3?

Answer 1

There's one small problem with all this. Here's what Great Weapon Master lets you do:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

The rules for bonus actions include this all-important clause:

You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one available.

So yes, every time you hit while your target is surprised, it will be a critical hit and trigger a bonus action attack from Great Weapon Master. But you can only ever make a single attack that way, because it requires a bonus action, and you can only take one bonus action each turn. And, as @Secesptius points out, if your second attack came from dual-wielding, that required a bonus action, so in that case, you won't be able to get a bonus action attack from Great Weapon Master at all.

For your specific questions:

1. Yes, if you surprised the target, they're surprised until the end of their first turn, so every attack will be on a surprised target.
2. Since they're attacks on surprised targets, each attack will trigger Assassinate.
3. Yes, since they trigger Assasinate, they are critical hits and trigger Great Weapon Master.
4. Yep, as before, they're surprised until the end of their first turn.
5. Therefore any further attacks you make would trigger Assasinate. Unfortunately, as discussed, you can't make any further attacks.
6. You still can't make more attacks, so there's no attacks to loop with.
7. Hiding would also require an action or bonus action, and you still don't have either of those.
8. They would still be surprised regardless of whether you hid.
9. Still out of actions to attack (and thereby Assassinate) with.
10. Same again.

Answer 2

No, there is no loop or bug that you could exploit - you get only one Action and one Bonus Action per turn

The first problem in your assumptions is the usage of two attacks.

If you attack with one light weapon you can make a Bonus Action to attack with another light weapon, which you wield in your other hand. This means that your Rogue can attack with one dagger as his action. If the target hasn't taken a turn yet this attack is done with advantage. If your Rogue hits and the target is surprised the hit is a critical hit.

After this attack you can make another attack as a Bonus Action with your second dagger, again as a critical hit because the target is still surprised.

The Great Weapon Master states (emphasis mine):

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

This means you get another Bonus Action with which you can attack.

But you only get one Bonus Action per turn. You can choose which one you want.

This Bonus Action attack could still trigger the Advantage part from Assassinate. It would still trigger the Critical Hit part, because the target is still surprised until the end of their turn. That means the Bonus Action attack triggers the part about you getting a Bonus Action to attack. But you have already taken a Bonus Action in that turn, so this is meaningless at this point.

There is no loop you can exploit. You have to carefully read the texts in regards to what grants you a Bonus Action and remember that you can only ever take one Bonus Action per turn.

The same applies to hiding.

Answer 3

First, what is "surprised"?

You are surprised if your DM determines that you were not ready for combat (usually when you were not aware of your attackers) when initiative is rolled. Note that someone already fighting an enemy cannot be surprised by an attack. Their attacker may be hidden to them (and get advantage), but they cannot be surprised.

If you were surprised, you cannot take actions of any kind (including bonus action and reactions) until the end of your first turn (at which point you are not longer surprised).

If you were not surprised, then combat occurs as normal.

So, if a dual wielding assassin rogue can surprise an enemy, they can get advantage and auto-crits on both attacks. However, you can only proc your sneak attack once on your turn. So, if you are level 3, you could do 2d4+4d6+dex damage with one, and then 2d4 with the other dagger.

Great Weapon master does not apply here for a number of reasons:

1. GWM only applies to 2 Handed Weapons, and weapons with the versitile trait. All weapons that qualify for one of these does not have the finness property, which allows you to use DEX instead of STAR. Using DEX for your attack is a prerequisite for triggering sneak attack.

2. GWM requires a bonus action to gain an extra attack. You already used your bonus action to make your offhand attack, and you are only allowed on bonus action per turn.

This means that not only can you not use GWM in this situation, there is no situation where GWM can give you more than one additional attack. There is no way for anyone to gain more than one bonus action.

Now, with skulker, you can sneak out and hide when you miss your attack. This will not happen with rogues assassinate feature. But even if it did, surprise only lasts one round, no matter what.