Seems to me that the Clay Golem should have a to hit of +9 with its slam attack?

Hm, I suppose technically CR9 and Level 9 are different, but if they were, you would be correct. Its proficiency bonus appears to be +3 though, the other issue being the only thing it is proficient in is its slam attack (and a language but irrelevant). Thus, you can't see if anything else displays a different bonus.

A creatures CR is what it's proficiency bonus is based on, not it's hit-dice like it is for player characters, so for the purpose of the clay golem is has a bonus of +3. You can find more on this in Chapter 9: Dungeon Master's Workshop - Creating A Monster - Monster statistics by Challenge Rating table (I've only got the digital version, so I don't know what page it is).

Yeesh, that hit point maximum reduction is nasty.

Clay Golem > Tarrasque 

If your a 17th level  Elf wizard you could make 6 clay golems a day with the true poly morph spell.  6 X 365 = 2,190.

Congrats, now you can slaughter the Tarrasque 

Hover Crafts! Sick I love Mazes and Monsters

huh, this would in fact not only beat a tarrasque, but would take absolutely no damage from one.  CR is a joke I guess.

Quote from Zelvier >>

A creatures CR is what it's proficiency bonus is based on, not it's hit-dice like it is for player characters, so for the purpose of the clay golem is has a bonus of +3. You can find more on this in Chapter 9: Dungeon Master's Workshop - Creating A Monster - Monster statistics by Challenge Rating table (I've only got the digital version, so I don't know what page it is).

 The problem with that is that a CR 9-12 (or level 9-12) should have a proficiency bonus of +4. 

:) seems fun to play as one

I'm thinking that it's speed is such that you or a party should be able to simply dance around it and use ranged attacks. 

Quote from elfcobra >>

Clay Golem > Tarrasque 

 YES IT IS

Clay golem immunities: Acid, Poison, Psychic; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Adamantine
Clay golems have immunity to frightening
Tarrasque Attack damages: Acid, Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Adamantine
Tarrasque immunities:Fire, Poison; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks
Clay golem Attack damages: Bludgeoning from magical attacks

so if you wait long enough the clay golem WILL eventually kill a tarrasque, but it would take forever because tarrasque has 676 HP and clay golem only deals 16 damage and the tarrasque's AC is 25 which means the clay golem would need to roll a nat 17 or higher to hit, meaning that a clay golem would only hit 15% of the time

Quote from Trewill91 >>

huh, this would in fact not only beat a tarrasque, but would take absolutely no damage from one.  CR is a joke I guess.

 not only that but it would heal from the swallow attack's acid damage

Yeah, the Clay Golem is an example of min-Max Monster, with a CR to reflect that. Huge strengths in the from of its immunities from a whole host of non-magical attacks and certain conditions. The rider effects on its slam attacks can also shut down Healing through the reduction of max hp. No matter how powerful you are, if your stuck in a room and your only option of attacks are non-magical it’s going to win.

However, this is counter-balanced by major weaknesses. If your weapon attacks are magical, it’s got a relatively low AC to hit. It also can’t fly, has a slow speed, and can only do damage in melee. In a closed space it can be scary, but in an open space, a person can just walk away from it, and if they have anything that can damage it from range, it becomes a piece of cake.

In conclusion, the Tarrasque (which is sadly not so powerful of a monster for its CR and history.) would not fair well if it was trapped in a room with the golem, because it can only engaged in a way which the golem is maxed for. But a Level 5 Wizard in a open space with ‘Fly’ can just hover out of it reach, and whittle it down with ‘Firebolt’ without fear of being harmed because the Wizard is engaged in a way the golem is minimized for. Heck, as long as the Wizard could find a place the golem could not reach, they could defeat the golem with Firebolt at level 1.

What is the point of this Haste effect? It gives up 2 attacks in order to make one extra attack on a subsequent turn? I can see it's defensive use, but on a creature with a 3 Int score that kind of tactical anticipation of a benefit seems out of character. It should at least increase the creature's speed for a round.

I thought that to, until I looked at it more closely. Over two rounds it will get the same number of attacks, hasted or not, because it can use its bonus action on the turn it it hasted itself for a slam attack.

Attacks over Two Rounds with including Haste Ability

Round 1: Action to Haste + Bonus Action to Slam= 1 Attack

Round 2: Action for 2 Slams + Bonus Action to Slam= 3 Attacks, for a total of 4.

In reality, its delaying one of its attack for the next round for a temporary AC boost, and having advantage on Dex saving throws. {May or may not be redundant with Magic Resistance, dependent on opposition it faces.)

Quote from Lasthope64 >>

I thought that to, until I looked at it more closely. Over two rounds it will get the same number of attacks, hasted or not, because it can use its bonus action on the turn it it hasted itself for a slam attack.

Attacks over Two Rounds with including Haste Ability

Round 1: Action to Haste + Bonus Action to Slam= 1 Attack

Round 2: Action for 2 Slams + Bonus Action to Slam= 3 Attacks, for a total of 4.

In reality, its delaying one of its attack for the next round for a temporary AC boost, and having advantage on Dex saving throws. {May or may not be redundant with Magic Resistance, dependent on opposition it faces.)

 Nice catch!

Yeah that's painful, heroes beware cause you soon shall be dead

Who would win? the strongest monster in dnd. Or one clay boi

A Clay Golem will kill a Tarrasque ten times out of ten