Quote from Hordus_Realmwalker >>

So question, stench, failure on the CON check means the player is poisoned. What does the poison do? Based on the my past knowledge (1st edition) I made the decision that it would cause disadvantage on "to hit/saves" until the ghast's next turn, at which point the player can attempt another save to become immune. Any thoughts?

 hover over the word "poisoned" in the abilities description, it will tell you exactly what it does, also read up on the status conditions before starting the game.

bro

"he kinda looks like vin diesel"

cant unsee that now

Quote from Detrimental >>

How do you come up with these? 

It sounds weird to advocate for an older edition, but it's worth picking up (or using your favorite search engine to find a "shared" pdf of) the D&D 4th Edition Monster Manuals. They're loaded with this kind of stuff, and they are frankly slightly better designed once you learn to parse the stat blocks. The monsters in 4e were designed to be easy to choose to balance for an interesting fight, with explicit roles detailed in the stat blocks (for example, the Ghast might have been given the sub-role "Leader" or "Brute", which is kind of self-explanatory). They also detail tactics for the creatures, making them easier to understand how to play as for the DM.

Don't try to use the powers and stats though, unless you feel comfortable doing the math to understand the conversion. I don't think there's any monsters yet released for 5e in a source book (meaning, not an adventure-specific monster) that don't have a 4e equivalent. 

This part is calculated incorrectly...

"Bite. Melee Weapon Attack: +3 to hit"

It should be +5 to hit... whether a ghast attacks with Strength of Dexterity. The damage is 2d8 + "3". So that should tell any powers that be everything they need to know about fixing this lingering errata.

And no, if a humanoid-turned-undead can be proficient with their previously non-existent "claws", they can be proficient with their "teeth" (which they've been using for a LOT longer in life before become undead).

That's brilliantly described, thanks Rick

The poison gives disadvantage on all ability checks and attack rolls. (Hover your mouse over the word poison in the description)

Quote from Angrygodofmilk >>

This part is calculated incorrectly...

"Bite. Melee Weapon Attack: +3 to hit"

It should be +5 to hit... whether a ghast attacks with Strength of Dexterity. The damage is 2d8 + "3". So that should tell any powers that be everything they need to know about fixing this lingering errata.

And no, if a humanoid-turned-undead can be proficient with their previously non-existent "claws", they can be proficient with their "teeth" (which they've been using for a LOT longer in life before become undead).

 This is not the case, according to both the books and the Errata.

But, do what makes sense to you in your own game.  A +2 bonus isn’t going to break anything, and it may be easier to remember one “to hit” for every attack (which is how most monsters work).

Your information for DCs is in the wrong order (in my opinion):

DC5 (very easy): attacks with a bite or a claw (you can see teeth and claws, makes sense)

DC10 (easy): immune to being charmed, exhaustion, and poisoned, resistance to necrotic damage, attacking the creature with poison has no effect (you know it's undead) 

DC15 (medium): its claw attack can cause paralyzation, additionally that it has darkvision out to 60' (you think it's a ghoul)

DC20 (hard): being next to the creature's stench may cause one to be poisoned and it may be difficult for a cleric to turn (you know it's a ghast)

DC25 (very hard): other undead in it's vicinity will also be difficult for a cleric to turn (you know what effect ghasts have on packs of undead and possibly the link to Orcus)

I know its a late reply, But in 5th edition, the poisoned condition depends on the type of poison used. As some cause you to fall unconcious, others deal damage over time, etc. But all poisoned creatures will generally have disadvantage to whatever they do.

To clarify:
Alcohol                 - Disadvantage
Potion of poison - Disadvantage, 3d6 damage at the start of your turn, every turn you are poisoned
Drow poison        - Disadvantage, Unconcious for duration if failed con save by X amount

Edit, nvm other peeps have answered this

Quote from Detrimental >>

Anybody else think he kinda looks like Vin Diesel?

Nah man, I think the ghast is played by Dwayne "The Rock" Johnson.

I agree, except that the check should be Nature, not Religion. The ghast's traits aren't really connected to any religion.

I use religion as a skill check against anything with the "undead" creature type.  It's more appropriate in my mind but each dm can decide on their own.

This is my table - your mileage may vary...

Skill Check Monster Type
Intelligence (Arcana) Aberrations, Constructs, Dragons, Monstrosities, secondary Fey
Intelligence (History) Giants, Humanoids, Goblinoids
Intelligence (Nature) Fey, Oozes, Plants
Intelligence (Religion) Celestials, Elementals, Fiends, Undead
Wisdom (Survival) Beasts

Pretty cool to see you back at this again after so many years. Your work is appreciated.

Note: Ghasts actually look like normal humans. DnD canonically had a Ghast city, from what I remember. Additionally, Ghasts retain their memories from life. 

Therefore, casting the Clone spell on somebody, killing them, and turning their corpse into a Ghast is a the most efficient method for creating clones of yourself.

This information is courtesy of MrRhexx on YouTube, take it up with that guy if I'm wrong.

Hey, does anyone know if the Turning Defiance ability applies to the spells like Create Undead? If not, what does it apply to? Thanks! 

Number of Attacks - I'm assuming 1 per round, since it does not have the MultiAttack attribute specified.

Multiattack

A creature that can make multiple attacks on its turn has the Multiattack action.

Nope, just to the Channel Divinity Feature Turn Undead, though Create Undead will probably turn a Ghast corpse into a raised Ghoul, if Create Undead can be used on undead corpses at all.

...

Quote from Angrygodofmilk >>

This part is calculated incorrectly...

"Bite. Melee Weapon Attack: +3 to hit"

It should be +5 to hit... whether a ghast attacks with Strength of Dexterity. The damage is 2d8 + "3". So that should tell any powers that be everything they need to know about fixing this lingering errata.

And no, if a humanoid-turned-undead can be proficient with their previously non-existent "claws", they can be proficient with their "teeth" (which they've been using for a LOT longer in life before become undead).

  Jeremy Crawford covered this in Sage Advice:

The bite attacks of ghouls and ghasts don’t appear to have proficiency bonus added in. Intentional or mistake?

Intentional; it’s a ghoul/ghast thing. They’re exceptionally bad at biting, compared to their claw attacks. Monsters sometimes have stat quirks like that.