Does casting a Subtle Spell count as surprising a target if target does not know you are caster?

Question

I understand you can't Sneak Attack with spells, something to do that both abilities scale thus game breaking. So that takes away one aspect of my predicament. However the Assassinate ability still gives Advantage over lower initiative targets, and according to Can an Assassin use Assassinate with ranged spell attacks?, an assassin with surprise will auto-crit with a spell.

If you have the Subtle Spell metamagic, can you still Auto-Crit with a spell attack after the start of the encounter if the target doesn't know you're a spellcaster? Does casting a Subtle Spell count as surprising a target if target does not know you are spellcaster?

For example: as an Assassin 3/Sorcerer 3, at start of encounter I shoot a target with a light crossbow and Hex spell, dealing 1d8+2 damage. Target has yet to see if I or any of my allies is caster. Next round I use Chill Touch at same target. Do I auto-crit and roll 2d8 cold damage +2d6?

Answer 1

No.

This does not work as you have described in your question. Surprise can only happen in the first round, and is based on stealth. It doesn't mean that it applies when the enemy is surprised to find out that you're actually a magic user. Additionally, surprise is a state; a character is either surprised or is not.

...If neither side tries to be stealthy, they automatically notice each other. Otherwise, the GM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.

If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends. A member of a group can be surprised even if the other members aren’t.

Your assassin auto-crit ability only triggers when you have the drop on an enemy and they have not succeeded on a Wisdom (Perception) check to spot you. During the first round of combat, you will have advantage against any enemy who is surprised, and any hits against them will automatically be critical hits.

In your scenario, you would need to cast your spell in the first round for this to apply, but I will note that your crossbow attack + Hex damage would be counted as a critical if the enemy were surprised.

Subtle spell does nothing to help your surprise in this situation.

Answer 2

No, because being surprised is a game state

Per PH189/SRD90, surprise is defined as

Any character or monster that doesn't notice a threat is surprised at the start of the encounter.

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.

Once they take their turn they are no longer surprised, and thus no longer subject to the assassinate feature.

Answer 3

No.

According to the PHB, the only time that surprise can be in effect is when a creature doesn't notice a threat:

Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.

The enemy knows that you're there and that you're a threat, so they won't be surprised. This question has more on why you can't be surprised during combat. Given that the wording of Assassinate requires that targets must be surprised, you cannot use it mid-combat.

Is turnabout fair play?

Consider this: how would you feel if the DM allowed an enemy to deal critical damage to you whenever it used a new or unknown ability in the middle of combat? Monsters, combatants, and even traps in the world of D&D are full of surprises, but the Surprise mechanic is narrowly limited, in order to limit its abuse.