D&D 5E Difference between Assassinate and Death Strike class features

[Ilbranteloth]

So, one of my players was looking at following the assassin archetype and asked me this. What's the difference?

Assassinate (3rd level+) - When you strike a creature that's surprised, it's automatically a critical hit. This would potentially stack with sneak attack, and a critical deals double damage.

Death Strike (17th level+) - When you strike a creature that's surprised, they get a Constitution save and if they fail, the damage is doubled. Assuming this stacks with sneak attack only it's the same thing, except worse (since the opponent gets a save to avoid). So does it stack with assassinate?

If it all stacks, a 17th level assassin with a rapier and a 20 DEX would cause 18(4d8) + 126(36d6) + 20 = 166 hp damage (average) assuming no magical bonuses. The amount varies slightly depending on how you do the math.

If it doesn't stack, then the 3rd level ability is actually better than the 17th level ability.

Considering the ability name, I can see that it would stack. But wow.

Ilbranteloth

 

[ZombieRoboNinja]

They stack.

And if you want to get crazy, look at the high-end poisons you can put on your blade (6d6 from purple worm poison), and consider that poison damage gets doubled on crits too.

 

[Tormyr]

Look at the assassin NPC. 24d6 if they hit you and you are surprised. Medic! And that is only a CR8 creature.

The +20 is incorrect. Critical hits only double damage dice not damage. So it is:

((1d8 + 9d6) * 2 +5) * 2
= (2d8 + 18d6 + 5) * 2
= 4d8 + 36d6 + 10
= 18 + 126 + 10
= 154

Of course, that is only if you surprise a creature in the first round, but there is a good chance of taking something out. your player is going to need more dice.

 

[Ilbranteloth]

Actually, I just picked up 733 mini six-siders for a steal ($5) so I think we're good there...

Ilbranteloth

 

[Chocolategravy]

They stack. And if you want to get crazy, look at the high-end poisons you can put on your blade (6d6 from purple worm poison), and consider that poison damage gets doubled on crits too.

I wouldn't double damage on things that are "behind" a save like those poisons are. Poisons that don't have a save and just add damage to the attack I would e.g. Basilisk's bite.