O.o

ctit for 24d6+3. No thank you sir. 

Poison damage is secondary. It doesn’t crit.

Yep, this is correct.  A crit here would be (1d6 x 2) + 3 for base dmg, then the ~24 (7d6) poison damage if failed saving throw after.

You mean 2d6 + 3, not (1d6 x 2) + 3. The latter is 3rd edition rules.

It's common to just double the rolled amount instead of rolling double the dice. Personally I've no clue from what additions they originate, I just know that doubling the rolled amount can be quicker than rolling more dice.  :)

It's common because many DMs carried the practice over from earlier editions either knowingly or because they confused the rules between editions. 5th edition rules (which DnD Beyond is concerned with) have you rolling double the dice. The benefit is if you roll a 1 on a d6 you have a chance to do more than 2 plus the modifier, the drawback is if you roll a 6 you have a chance to do less than 12 plus the modifier for damage. If you have to roll 2d6 for a sneak attack or 12d6 for dragon breath you don't roll once and multiply it by the number you're supposed to roll. I find it simpler because it is consistent with rolling everything else (where else do you multiply?) and also because the first roll doesn't make or break the whole thing. I have a bag of 36d6 dice I use for big damage. I don't find it tedious at all, as a matter of fact I think it is more of a fun thing when you get to roll more dice. It really inspires the person rolling them whether it is an assassin rogue getting that double sneak attack or the DM with a nasty dragon's breath.

Does the assassin get to hide as a bonus action? Seems strange that the uber-rogue wouldn't get that ability...

Why is this challenge rating 8? they're not that good 

Played properly, they are definitely CR 8. They have very high stealth and acrobatics (making them difficult to grapple). They have high damage output when utilizing their sneak attack properly, and even when not their poison is hard to save against and applies to every single hit. An unlucky target, even without getting sneak attack'd, would take 60 damage per round from short swords.

makes sense thanks for the info

I had five level 3s in one of my games and they were supposed to go kill this assassin but what ended up happening was one player went by himself while everyone else drank at the tavern... he was knocked unconscious I think the crossbow attack one shoted him but luckily two of his mates stepped outside and saw him, then they unleashed their pet demonic gnoll on the assasin. 

Quote from Brynnan >>

Poison damage is secondary. It doesn’t crit.

 
Where do you get that from?  Can you cite the rules?
All I see is: "If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice as well."

In this case the attack involves both the Sneak attack and the Poison as "other damage dice"

I think it's because it's conditional to a saving throw so not part of the attack itself.

Taking half is conditional; they take some damage no matter what and it's still doubled on crit.

More discussion around this here: https://rpg.stackexchange.com/questions/58118/how-does-extra-damage-work-for-critical-hits

I'm of the opinion that the poison wouldn't be doubled, but it's up to you and your DM in your games!

To each their own but I would hardly call it open to interpretation.  Critical Hits read "If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice as well."

The damage dice of the poison are involved in the attack, even more so than Sneak Attack since the poison damage is literally part of the attack.  It having an additional saving throw isn't relevant.

When a player is hit by this attack, they take poison damage.  They have a chance to take half, but the poison damage is part of the attack regardless.

It's confirmed by Jeremy Crawford that poison does NOT apply to critical hits when poison is added to a weapon if there's a save involved, "Any damage dice delivered by a critical hit—as opposed to a saving throw—are rolled twice." 

Remember to include those sneak attack dice in your Crit. damage roll.

This guy must really do his job well.  If he gets the jump on them, his damage output will absolutely melt a 50 HP knight or 65 HP Bandit Captain, even they succeed the their CON saves.  A 10 HP noble doesn’t stand a chance.

It looks like there are some Assassin options missing from the Compendium here. I have MTOF on Roll20 and it has Assassin of Baphomet and Assassin of Levistus. I don't see those listed here in the Monster Compendium. What am I missing?