The rules for the Assassin (a Roguish archetype) in the latest edition of Dungeons & Dragons include an extremely powerful ability: Assassinate. It’s the signature ability of the class, and it can be very effective. A reminder of what it does:

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.

However, as it uses the Surprise rules – and those rules were different in earlier versions of the game – I’ve seen many people be confused about how it works. This article discusses the rules and their interpretation.

The starting point is Jeremy Crawford’s explanation of surprise, from Sage Advice November 2015:

The first step of any combat is this: the DM determines whether anyone in the combat is surprised (reread “Combat Step by Step” on page 189 of the Player’s Handbook). This determination happens only once during a fight and only at the beginning. In other words, once a fight starts, you can’t be surprised again, although a hidden foe can still gain the normal benefits from being unseen (see “Unseen Attackers and Targets” on page 194 of the Player’s Handbook).

To be surprised, you must be caught off guard, usually because you failed to notice foes being stealthy or you were startled by an enemy with a special ability, such as the gelatinous cube’s Transparent trait, that makes it exceptionally surprising. You can be surprised even if your companions aren’t, and you aren’t surprised if even one of your foes fails to catch you unawares.

If anyone is surprised, no actions are taken yet. First, initiative is rolled as normal. Then, the first round of combat starts, and the unsurprised combatants act in initiative order. A surprised creature can’t move or take an action or a reaction until its first turn ends (remember that being unable to take an action also means you can’t take a bonus action). In effect, a surprised creature skips its first turn in a fight. Once that turn ends, the creature is no longer surprised.

In short, activity in a combat is always ordered by initiative, whether or not someone is surprised, and after the first round of combat has passed, surprise is no longer a factor. You can still try to hide from your foes and gain the benefits conferred by being hidden, but you don’t deprive your foes of their turns when you do so.

One of the notable things to take away from the explanation of surprise is this: You can’t surprise a creature if it has spotted (or heard) any foe. Not just you. If you’re hidden, but your dwarven fighter friend is clanking away and rolled a 0 for their Dexterity (Stealth) check, you’ll still be hidden (and can gain advantage against targets that haven’t spotted you), but because the foe spotted your friend, it isn’t surprised. Surprise is a state that individual creatures can be in; it’s no longer the case that the entire opposition is surprised or not.

The next thing is that surprise ends once a creature’s first turn in the initiative order has finished. This has some unfortunate implications to the assassin who rolls low for initiative. Those high-Dexterity creatures who rolled better initiative than you? They’re not surprised any more by the time you act. Although the weapon of warning (can’t be surprised, advantage on initiative rolls) is very good at frustrating assassins, it’s also the assassin’s friend. Advantage on initiative checks? Yes, please!

Given all of this, the Assassin has one major problem using Assassinate – the other members of the party. Those dwarven fighters just keep causing you to lose surprise, don’t they!

Back in AD&D, the entry for elves and halflings read like this:

If alone and not in metal armor (or if well in advance – 90′ or more – of a party which does not consist entirely of elves and/or halflings) an elven character moves so silently that he or she will surprise monsters 66 2/3% (d6, 1 through 4) of the time

That suggests the solution. All you need to do is adventure alone! Or adventure by scouting ahead – 90 feet or more – in front of the party. (Talk to your DM about how far you need to be ahead of the party to be considered “on your own”). Of course, the trouble with doing so is that your friends can’t help you when you get into trouble…

A related issue that often comes up in play is the player who announces “I attack” and expects to surprise the opponents (and thus get to attack before anyone else). The transition from negotiation to combat (or exploration to combat) is not explicitly handled as an exception the rules. Thus, if a player wishes to attack first before anyone can react, this is not possible. Upon a player indicating their intention to attack, move into the combat sequence – determine surprise, and determine initiative. Yes, the player might end up attacking last; the initiative roll indicates that he’s not as fast as he thinks, and everyone else has reacted.

If you’re feeling kind, you might allow a character who attacks whilst in the middle of a negotiation to make a Charisma (Deception) check against the passive Wisdom (Insight) scores of everyone else; creatures that don’t perceive the character’s attentions could potentially be considered surprised. However, this is a non-standard ruling and should be used only if you feel it enhances your game.

Although rare, it can happen that a creature is unaware of the existence of enemies, yet is not surprised and gets to act before its foes. In this case, I would have the creature acts on its initiative count as normal, doing whatever it was doing before the combat sequence started.