So here's this Giant Enemy Crab

[EyethatBinds]

The Seige Crab from MM3 basically is crying out for this to happen, but I need to add some form of vulnerability to crits where they do x3 for x2 weapons and x4 for x3 and so on. If I do this, should I also drop the CR of the monster slightly? It's a CR14, but this weakness should probably be a drop of some kind.

[KillianHawkeye]

Sounds good. You might wanna have an NPC drop hints that they can "Shoot the core for MASSIVE DAMAGE!!!" :smallwink::smallamused:

[Jellyface]

If the PCs may be able to learn about its weakness then I would drop its EL instead of its CR.

[Diarmuid]

Unless the party is optimized for crits or have done ability to make changes (gear, spells, feats, etc) this isn't really all that much of a change to the monster. Not enough to warrant changing a the EL/CR appreciably.

[Grod_The_Giant]

Unless the party is optimized for crits or have done ability to make changes (gear, spells, feats, etc) this isn't really all that much of a change to the monster. Not enough to warrant changing a the EL/CR appreciably.
Agreed. Now, if you were to allow some sort of special called shots...

[Flickerdart]

I'd implement some sort of weakness like "while prone, this creature takes double damage" which is a lot more reliable but still tricky to pull off, given how high the thing's modifier against tripping is.

[Jellyface]

I'd implement some sort of weakness like "while prone, this creature takes double damage" which is a lot more reliable but still tricky to pull off, given how high the thing's modifier against tripping is.

Since it's shell has hardness 15 and hp 30 you could apply that to the legs, and if they cut two of the three on one side it's prone or disabled (except for taking damage for acting).

Since the occupants can go in or out of it as a standard action as long as there isn't the force field or it's collapsed, then maybe a medium and smaller creature can attempt to get to the hatch, if they come from the front they provoke an attack of opportunity and if they come from behind they don't.

[supermonkeyjoe]

Isn't that the monster that has a compartment for passengers? Maybe have the controller and the crabs HP pooled but any successful attacks on the controller do double damage?

IIRC the compartment can be opened and closed giving either cover or total cover, excellent for giving "Shoot the crab while its core is exposed" action.