When the spell states that you take the form of "any creature with a challenge rating equal to your level or lower", does this mean the players class level, spell-caster level, or total level from all classes. Like would a player with 17 levels of Druid and 3 levels Barbarian be able to become a CR of 17 or 20?

I think it goes off of the players total level, so in your example, they would be able to become a CR 20.

Can you shape change into a warforge.

Quote from WundShadow >>

Can you shape change into a warforge.

depends on the DM's decision of whether they would be considered a construct or something more unique I suppose. But you would still be an "average example of the creature".

So then.... a lvl 20 can potentially shapechange into an Ancient Brass Dragon.... given that they've seen one before?

well.... I'd be more interested in being able to shape change into a Adult Bronze Dragon.... or, if we were in the ocean.... a leviathan... good god this would be epic

Quote from Tawelwch >>

So then.... a lvl 20 can potentially shapechange into an Ancient Brass Dragon.... given that they've seen one before?

 

well.... I'd be more interested in being able to shape change into a Adult Bronze Dragon.... or, if we were in the ocean.... a leviathan... good god this would be epic

 As I am reading it - rules as written - at level 17, when you can gain access to this spell, you could Shapechange into "any" Adult Dragon (assuming you had seen the type of dragon before). At least of the Chromatic and Metallic standards. At level 20, the best you can hope for is an Ancient White or Brass Dragon as the rest have a challenge rating of 21+ and that gets into "epic levels" (assuming any DMs use that kind of 21+ progression)

Has anyone used this spell in a game? It seems to me that the concentration requirement would make it almost useless. At the level that this spell would come available, the damage exchanged in combat and the number of hits the new form would take would end the spell in a round or less. Given that the spell costs an action, you may not even get to take a single turn with your new form and accomplish nothing more than burning a level 9 spell slot. Are DMs table ruling around the constitution saves required after taking a hit with an active concentration spells?

Quote from 111ragnarok >>

Has anyone used this spell in a game? It seems to me that the concentration requirement would make it almost useless. At the level that this spell would come available, the damage exchanged in combat and the number of hits the new form would take would end the spell in a round or less. Given that the spell costs an action, you may not even get to take a single turn with your new form and accomplish nothing more than burning a level 9 spell slot. Are DMs table ruling around the constitution saves required after taking a hit with an active concentration spells?

I had used it, and is one of my favourite spells in the game, because you are making the saves using the new form's saving throw proficiencies, so let's put an hypotetical lets say you transform into an ancient brass dragon, that has a +13 to con savings, you would have to receive more than 28 points of damage for the dc to be higher than 14 just to make the dc for you to be able to fail it, and taking into account that the average damage a terrasque, a cr 30 monster, makes is 28 i would argue the concentration aspect is almost irrelevant.

This spell combines very well with the War Caster feat as it gives an advantage on the save. That being said, as the above poster said, you are using the save of the new form, so just make sure whatever form you are shifting into has a high con and go wreak havoc on your enemies.

Also, point of note, just change into a dragon of Legendary status. You cannot use lair or legendary "actions", but it says nothing about legendary "saves" and most of the upper tier dragons have 3 of those.

"Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead."

Note that the verbiage of the phrasing means you can attempt your Con save and IF you fail it, just use one of the Legendary Resistances. 

In theory you should last at least 3 or 4 rounds as the creature in question (if it has Legendary Resistance) and in that time, you should be able to mess some stuff up. Especially if you roll well on your breath weapon recharge.

I think the Legendary Resistance comes under the category of Legendary Action, because a legendary action is basically just an action that a creature can do whilst it is someone else's turn. Also it has "Legendary" in the name.

If the intent was to include all things legendary I believe it would have been writen that way. As writen, it does not say you cannot use a creatures legendary saves, but it does say you cannot use its legendary actions or lair actions. 

Each of these things is a specific difference and considering the WotC lawyer speak writing of spells and actions and the various errata that they change over time (learned over years of writing the rules on Magic cards), I do not believe there is an intent to be vague. 

Should they change the rules or release something 'official' in regards to this, then I would abide by that, but at the end of the day, it still all comes down to what the individual DM decides works or doesn't work.

Quote from CasuallyChallenged >>

If the intent was to include all things legendary I believe it would have been writen that way. As writen, it does not say you cannot use a creatures legendary saves, but it does say you cannot use its legendary actions or lair actions. 

Each of these things is a specific difference and considering the WotC lawyer speak writing of spells and actions and the various errata that they change over time (learned over years of writing the rules on Magic cards), I do not believe there is an intent to be vague. 

Should they change the rules or release something 'official' in regards to this, then I would abide by that, but at the end of the day, it still all comes down to what the individual DM decides works or doesn't work.

 Well put, I stand corrected.

Would the material component of this spell be replaced by a spell focus since it doesn't say that the circlet is consumed?

It doesn't sound like the circlet would be consumed, BUT if the spell specifically states a component and a value like this I am confident a spell focus isn't enough to cast and you would be required to wear the 1500g + jade circlet at the time of casting the spell.

well that depends .. there are 2 cr 30 monsters tarrasque avarage on attacks is 24 28 32 so dc 12 14 16 ... all almost guaranteed pass with the stat you mentiond. Tiamat however the other cr 30 monster has an avarage of 28 24 32 67 88 77 91 and 72 ... jeah dragon breath attacks are almost guaranteed to make you drop this ... (altho if it hit you without this you and your d8 hit die would probably loze most if not all hp) this goes for all dragons and all creatures with breath weapons or dmg dealing special abilitys ... tarrasque doesnt have this since it is more of a impenetrable defense monster ... so you kinda used a bad example

Dragon Breath Weapons are NOT a guarantee to drop this because you forget that you can save for 1/2. A failed save always sucks, but I'd be glad to have this up as a damage sponge if nothing else in that case (because you'd lose the concentration after taking the damage).

But if you save against the BW? That 88 damage just became 44 and the Concentration check DC is 22. Not a gimme, but absolutely passable. 

This spell seems to me to be a worse true polymorph.

Quote from wakosama >>

Would the material component of this spell be replaced by a spell focus since it doesn't say that the circlet is consumed?

You always have to use the material component, rather than just a focus, if the material component has a cost; not only when it's consumed by the spell.

Would it be possible to only half transform into a monster?

No.