"Reactive. The marilith can take one reaction on every turn in a combat."

How is reactive different from being able to take a reaction on every turn anyway, which everyone can do, no?

I think the difference is between "turn" and "round."  Most characters get one reaction per round, which they can use on anyone's turn, but the marilith can get one each turn if they wanted, so multiple reactions in one round.

So basically, as it says, the marilith can just add 5 to its AC against every PC's melee attack on their turn, unless it takes a different reaction on that turn, such as an opportunity attack when the PC moves out of range.  Turn & round, a subtle difference but I think you're right.  Thanks.

But it can only use Parry on *one* attack in each turn, so if a character attacks twice in a single turn, the second attack will be against the Marilith's normal AC of 18.

You are correct; the difference is between round and turn; it's essentially able to parry a lot. 

Also that means multiple attacks of opportunity in a round. All those damn arms.

Makes the maralith quite an engaging opponent. A couple of these against a melee focused party is fun. If you boost the stats of them and give them magical swords - flame tongues for instnins - suddnsud they are quite a threat. 

Also makes them great to shapechange into. Counterspell every turn. <3

Was about to argue, and then looked it up on Sage Advice. Holy shit. Now I know what to throw at my party what thinks they're so cool.

How would this thing reproduce if it had a male snake bottom

Damn she gives a whole new meaning to curvy

Same way regular snakes reproduce I’d imagine

Shapechanging Bladesingers can add their int mod to AC, Concentration checks, and the damage of all 7 attacks c:<

Interestingly, shield can stack with parries on subsequent turs. If Bracers of Defense are allowed to fit due to the armor being natural, this beast can reach 18+2+5+5+5 = 35 AC.

If you have an aspiring wizard in your party, be careful about this one ;)

Do demons need to eat? I feel like no, but I can’t think of anything saying that, and also Yeenoghu’s entire goal is to eat reality, so?

Pretty sure they don't have to eat. They just enjoy the taste of humanoid flesh and all that "Chaotic Evil" stuff.

Also, Yeenoghu wants to destroy the world and then rule the wasteland, not eat reality. I'm pretty sure anyways.

Quote from ClericalKenku >>

Do demons need to eat? I feel like no, but I can’t think of anything saying that, and also Yeenoghu’s entire goal is to eat reality, so?

Quote from TheDarkestDemon >>

Pretty sure they don't have to eat. They just enjoy the taste of humanoid flesh and all that "Chaotic Evil" stuff.

Also, Yeenoghu wants to destroy the world and then rule the wasteland, not eat reality. I'm pretty sure anyways.

 Yah, Darkest is completely correct that Celestials and Fiends don’t benefit from eating traditionally, but do it for pleasure.

That being said, demons spawned from the abyss have the special ability to devour the spiritual essence of their victims, and (over a long period of time) incorporate into themselves.  This “you are what you eat” factor is why they often look like chimeras of humanoids, beasts, and other monsters.

So, an abyssal lord could very well decide to devour worlds’ worth of souls on mass, in a longterm bid to grow in power.

Just use Shocking Grasp and you win ;)

I did this to a monk that had Reactive; baited out an AoO from him and slapped him with shocking grasp while the reaction wasn't available 😁

how do you kill one with a party?

I never noticed she could TELEPORT, holy shit she's scary.