I'm new to DMing and creating monsters, and I created my first: https://www.dndbeyond.com/monsters/1993483-giant-ambush-crab

I want my adventurers to run into this beauty as their second combat encounter, to start teaching them that nature is dangerous. I added a taste of the Action Oriented Monster idea just for flavor.

The players are all first-timers, and the PCs will all still be 1st level (though probably with a bit of a HP buff from an Aid spell from a villager). They are a paladin, a druid, and a fighter (Interception fighting style). 

I'd like this encounter to be very challenging and fun, but I really would like to avoid killing them. That's part of the reason I designed the Villain Action to require dropping any grappled players -- gives me a bit of an out in terms of helping the players avoid getting stuck.

Any feedback? Should I make it harder? Easier? Any ideas to make it more fun?

The druid will cast animal friendship and most likely convince the crab not to attack.

From melee classes, the fighter and paladin properly specced have a +5 to hit and will do 6 damage per hit.  Assuming 50% miss rate they will do 6 hps per turn to the crab.  A properly specced druid will have +5 to hit and will do 6 hps per turn to the crap.  So half of 18 damage = expected 9 damage per turn.  The crab is expected to have 3 to 4 rounds of up time before dead.  During which, time the crab will be trying to hit fighters at AC 18 with their starting gear and it has a 25% chance to hit the fighters with its to hit, so it would be expected to do 1 damage per turn.

It depends on starting gear and how the players fight.  If you want a longer fight let the crab go into its shell at 50% and add +5 to its armor and heal 2d6 hit points twice in the fight.  If you want a deadlier fight give it multi-attack OR when it has a character that is restrained it drags them into the water to drown them.  The extra crabs will have a 10% chance of hitting the players and only do 1 hp of damage, I'd change them or reduce the number and make them do damage like a minion but have 1 hp.

Quote from dadacomputer >>

I'm new to DMing and creating monsters, and I created my first: https://www.dndbeyond.com/monsters/1993483-giant-ambush-crab

I want my adventurers to run into this beauty as their second combat encounter, to start teaching them that nature is dangerous. I added a taste of the Action Oriented Monster idea just for flavor.

The players are all first-timers, and the PCs will all still be 1st level (though probably with a bit of a HP buff from an Aid spell from a villager). They are a paladin, a druid, and a fighter (Interception fighting style). 

I'd like this encounter to be very challenging and fun, but I really would like to avoid killing them. That's part of the reason I designed the Villain Action to require dropping any grappled players -- gives me a bit of an out in terms of helping the players avoid getting stuck.

Any feedback? Should I make it harder? Easier? Any ideas to make it more fun?

 This looks pretty good so far, and definitely good for your first homebrew monster! Keep in mind that if the PCs are doing too well or too poorly, you can change the HP on the fly.

I think that it will probably be an easy fight. It really depends on how focused on combat your PC's are, but this encounter should take 2-3 rounds if they are all rested and they will not take much damage from the Crab.

I would bump the to hit to +4, the damage to 1d6 + 2 and have it attack twice in a round to make it a bit more challenging.

The crabs are a fun idea, but they are going to be mostly useless on the fight. With a +0 to hit and 1 damage, they are not going to contribute on the fight.

If you feel that the fight is too easy, you can always add some giant crabs coming from the bog/swamp or whatever they are, mid fight, and make they ran away if the big guy fall.

You never mentioned how many players/characters vs this crab. Nonethless, I would either remove subsonic call for up to 6 (1d6) more coming out, else limit it to just 2 more...and even then, depending on how characters are performing against the original, I'd give them all a chance (decision) to consider to continue fighting....many monsters, or players, won't fight to the death if they have a way out. So, if players continue fighting, though you offered them an out, you can't be blamed as a DM.

First level is inherently very swingy, but with only a +3 attack bonus and one attack there's a reasonable chance it won't even do any damage before being defeated.

The useful thing about Legendary Actions in a monster like this is that you can use them as difficulty adjusters in the middle of the fight. If the party is struggling, you don't have to use it at all. If they're steamrolling it, you can fire it early to keep the fight challenging. I'd also recommend not rolling a d6 for the crabs but just picking a number in that range based on how the fight is going.

I agree with making the Attack a Multiattack, possibly by making one claw and one bite. I like that you constrained the bite to someone who is grappled, but it probably needs some "teeth". Might consider upping the damage die to a d8 or adding another +1 or 2. 

One aspect that might be being overlooked is the 20ft. drop (7pts avg. bludgeoning dmg) from the crab springing its ambush and that the PCs will most likely be surprised for the first round of combat. (Also, there is the not-to-insignificant chance that they will end up prone after the fall. AKA: Advantage on claw attack) Which means that when the PCs aren't surprised, the minions get summoned and it's swarm time. (Don't mind me wringing my hands and grinning.)

Is it possible to kill a PC with this, yep. Do the conditions have to be *just right*, yep. There's a good opportunity for someone to get a new set of PC wings if run "properly".

Wow tons of great advice! Thanks, I'll report back how it goes.