Quote from Kerethos >>

"Depends on your DM": the spell.

But remember this spell only adds onto existing terrain. You can change what it looks like or appears to be made of, but the previous terrain is still there. You can make a tree look like a dead tree, or a different tree, or turn it into adamantite, but you can't remove terrain.

Some examples:

• You alter a house to have a hole in the wall. That hole is full of the non-altered wall, as it is only a hole in the illusion. The original terrain is still there. Likewise, adding a window would just fill the window with the unaltered wall. The old terrain is still there, you are just covering it.
• You turn an open field into a swamp. That swamp is no deeper that the old field, as that becomes the bottom of the swamp.
• You make an adamantine wall around a town. Wherever the wall would conceal anyone, when you cast the spell, is now a hole. The spell does not conceal creatures.
• You try to turn a house into glowing hot lava/metal. This fails where the change would conceal a creature, so any surface that would conceal a creature remains unchanged. Everything might still catch fire though, because most of the floors are now made of glowing hot lava/metal.
• Your turn a road into a river. The bottom of the river is the road, you've just added a river on top. Thus the river is only as deep as you raise the surrounding terrain.
• You turn a river of lava into ice. The ice is cold, you can walk on it, but there's still lava underneath it. Make a hole in the ice and things get hot again.
• You make a tree. The tree can be up to 1 mile high and made of whatever you want (like adamantite, gummy bears, or paper). You cannot awaken this tree if you make it more than huge in size. If it is huge it can be awakened, but will disappear at the end of the Mirage Arcane duration.
• You turn a snowy valley into a green valley in full bloom. This is added on top of the snow, it does not replace the snow, and the air is still cold.
• You make an illusory bridge over a chasm. The bridge remains in place for 10 days, unless destroyed, so make it out of something very durable.
• You are at the top of a mountain. It is a mile long drop to the ground bellow. You can turn it into a mile long slope instead, and ride a shield down said slope for fun. Or add stairs, if you're going up or want to walk down.
• You turn a dune filled desert into a nice tempered forest around a lake. That lake is no deeper than the dunes allow for and the climate remains unchanged. You can swim in the lake, you can drown in the lake. Beneath the lake is still the desert dunes.
• You make an illusory rock and throw it at someone. It acts like any rock. If a creature has truesight it will see that it is an illusory rock, but it is still solid and will act like a solid rock. They just know it's an illusion and can see through it, but it still hits them like a solid rock and acts, feels, smells and tastes like a solid (though transparent) rock.

Final thoughts

I'd consider trying to turn a whole city into lava/glowing hot metal and watch everyone burn... if I was evil. Good thing I'm totally not. Go team! Or make a 1 mile high adamantine tower to live in, with a lava moat. Or taunt my enemies by making a giant dick rise on the horizon. The sun behind it, so our enemies knows we'll fight them in the shade... of the peen. But that's just me. You do you.

For damage I'd use no more than the recomended 12d6, from the "Create a Spell"-section of the Dungeon Master's Guide, for stuff like hazards created by this spell, such as lava, hot metal, or an acid lake to name some hazards.

This is spell really depends a lot on your DM, so discuss its use with them before you pick it and see how they are okay with running it. Otherwise you risk turning a whole session into nothing but a long discussion on the rules. That is much better to have before or after the game, if you can, so you don't stop the game for everyone who didn't come for a long rules discussion. I hope this was of use to someone. :)

 
I think you might be confused on what “conceal creatures” means. Conceal means to turn invisible. If I make a wall in front of a creature the spell wouldn’t spontaneously fail. Better wording should be used here. 

One of the examples in the spell description is that it could change a road into a crevasse (a big hole in the ground), so it definitely can remove terrain. It has to remove the ground to create the hole. The only way this could be adding is if you were lifting the whole ground up by the depth of the hole and leaving the part where the hole should be untouched.

Not quite.  Actual wording- "could be made to resemble" a crevasse or other impassable terrain.  Later in the spell it describes the addition or subtraction of difficult terrain as the only game mechanic that is affected by this spell.  So my DM ruling, fwiw, has been this:

Can't remove existing terrain.  Can alter existing terrain sufficiently to negate difficult terrain or add it.  Can add quasi-real terrain or structures that bypass hazards.  Can add structures that provide cover (game mechanic term not prohibited by spell description- lot of above comments are mixing up concealment and cover).  Can't create environmental effects that would conceal creatures, like fog, snow, heavy rain, etc.  Can't disguise large groups of creatures (ie not a Supersized Seeming).  Can't alter existing structures in anything but appearance.  Can't make items- only terrain and structures.  Can't cause damage- a lava river you create will only be difficult terrain, even though its the genuine article in every other way.

Something that people seem to be missing is that the range of this is a SQUARE, not a CUBE, meaning that while the height and depth are kinda vague it stands to reason that you can still make structures that are one or two stories max but that part would be a question for the DM.

So no mile deep pit or mile tall dragon statue. (You could still make a mile long serpent dragon statue though!)

This is a great spell to duplicate with Wish as it will reduce the 10 minute casting time to 1 action, allowing it to be used in combat to completely reshape the battlefield.

Encase every enemy in in sight with an adamantine dome filled with lava. GG.

I had a question about that actually. The range is listed as sight with an area of effect of one mile right? So does that mean you could effect a one mile area anywhere you can see? If for example my wizard in on a mountain and can see town in the valley bellow, could they cast this on that town?

Yes.  Any one mile square area ( as Mr Fugums notes, not a cube.  So you can't go totally crazy on height) within range of your sight.  Does not have to be a mile away from you.  Bring a spyglass (if I was your DM I'd say detail/accuracy are otherwise difficult at that range) and have at it!

Be an illusionist wizard. Be unstopple. Make everything into adamantine traps.

Quote from DIemortalscum >>

Quote from Kerethos >>

"Depends on your DM": the spell.

But remember this spell only adds onto existing terrain. You can change what it looks like or appears to be made of, but the previous terrain is still there. You can make a tree look like a dead tree, or a different tree, or turn it into adamantite, but you can't remove terrain.

Some examples:

• You alter a house to have a hole in the wall. That hole is full of the non-altered wall, as it is only a hole in the illusion. The original terrain is still there. Likewise, adding a window would just fill the window with the unaltered wall. The old terrain is still there, you are just covering it.
• You turn an open field into a swamp. That swamp is no deeper that the old field, as that becomes the bottom of the swamp.
• You make an adamantine wall around a town. Wherever the wall would conceal anyone, when you cast the spell, is now a hole. The spell does not conceal creatures.
• You try to turn a house into glowing hot lava/metal. This fails where the change would conceal a creature, so any surface that would conceal a creature remains unchanged. Everything might still catch fire though, because most of the floors are now made of glowing hot lava/metal.
• Your turn a road into a river. The bottom of the river is the road, you've just added a river on top. Thus the river is only as deep as you raise the surrounding terrain.
• You turn a river of lava into ice. The ice is cold, you can walk on it, but there's still lava underneath it. Make a hole in the ice and things get hot again.
• You make a tree. The tree can be up to 1 mile high and made of whatever you want (like adamantite, gummy bears, or paper). You cannot awaken this tree if you make it more than huge in size. If it is huge it can be awakened, but will disappear at the end of the Mirage Arcane duration.
• You turn a snowy valley into a green valley in full bloom. This is added on top of the snow, it does not replace the snow, and the air is still cold.
• You make an illusory bridge over a chasm. The bridge remains in place for 10 days, unless destroyed, so make it out of something very durable.
• You are at the top of a mountain. It is a mile long drop to the ground bellow. You can turn it into a mile long slope instead, and ride a shield down said slope for fun. Or add stairs, if you're going up or want to walk down.
• You turn a dune filled desert into a nice tempered forest around a lake. That lake is no deeper than the dunes allow for and the climate remains unchanged. You can swim in the lake, you can drown in the lake. Beneath the lake is still the desert dunes.
• You make an illusory rock and throw it at someone. It acts like any rock. If a creature has truesight it will see that it is an illusory rock, but it is still solid and will act like a solid rock. They just know it's an illusion and can see through it, but it still hits them like a solid rock and acts, feels, smells and tastes like a solid (though transparent) rock.

Final thoughts

I'd consider trying to turn a whole city into lava/glowing hot metal and watch everyone burn... if I was evil. Good thing I'm totally not. Go team! Or make a 1 mile high adamantine tower to live in, with a lava moat. Or taunt my enemies by making a giant dick rise on the horizon. The sun behind it, so our enemies knows we'll fight them in the shade... of the peen. But that's just me. You do you.

For damage I'd use no more than the recomended 12d6, from the "Create a Spell"-section of the Dungeon Master's Guide, for stuff like hazards created by this spell, such as lava, hot metal, or an acid lake to name some hazards.

This is spell really depends a lot on your DM, so discuss its use with them before you pick it and see how they are okay with running it. Otherwise you risk turning a whole session into nothing but a long discussion on the rules. That is much better to have before or after the game, if you can, so you don't stop the game for everyone who didn't come for a long rules discussion. I hope this was of use to someone. :)

 
I think you might be confused on what “conceal creatures” means. Conceal means to turn invisible. If I make a wall in front of a creature the spell wouldn’t spontaneously fail. Better wording should be used here. 

 You are mistaken about this (or at least your ruling is not clearly supported by the books).  The word conceal is used in the PHB a few times, like:

• in describing stealth ("Make a Dexterity (Stealth) check when you attempt to conceal yourself from enemies"),
• in the Oath of the Ancients Turn the Faithless power ("if a creature's true form is concealed by an illusion or shapeshifting"),
• in the Warlock's Witch Sight invocation ("...see the true form of a shapechanger or a creature concealed by illusion or transmutation magic") and
• in the section on Cover, under total cover ("A target had total cover if it is completely concealed by an obstacle")

It is not used in the spell "Invisibility" at all though.  Stealth conceals you but doesn't make you invisible.  Transmutation magic can conceal you, but it never makes you invisible. An obstacle, by RAW, can "conceal" you, but that isn't the sort of invisibility you mean.

So I'm a little confused by this interpretation of conceal.  If I can't make a wall because it "conceals" a creature then I can't make a building or a tree or a hill or literally anything in a place with more than 1 living being in it since it will always be concealed from some other creature, be it ant snake or human.   3 ants where you want to put that hill or tree?  nope.  fails since they are now concealed. 
The spell says it's possible for creatures to removes rocks and twigs from the illusion as well, which means the illusion can be moved and is pretty much real in every other way.  Does that mean if I tried to pile rocks and sticks into a wall it would then vanish???  Or is it even stranger in that it literally cant' conceal any creature at all so no matter what, living beings show through the illusion at all times?   I'm not sure but that sentence is terribly vague and that interpretation makes absolutely no sense if you examine it with any sincerity. 

I feel like the spell can't conceal just means it can't make a creature invisible as you can do with the landscape features.  You can make a forest completely invisible and make it look like an open plain.  but the creatures in the forest wouldn't be turned invisible.  you would instead have an open field with a bunch of very confused forest animals.   If you use the definition of concealing to mean anything that hides sight then the spell becomes unusable. 
  
The intent of the spell to me seems to be just to alter the terrain in any way you want but you can't add or alter creatures in any way.  The illusion can't incorporate a creature into itself but creatures can still interact with it.   Altering a creature's appearance is against the rules as is trying to turn it into anything, but putting trees around it or walls or a hill is fair game because the creature is not the target of the alterations caused by the spell.  

a golem*

She is a beautiful woman

I think the best way to think of this spell is like reskinning an area of terrain. It makes me think of when people talk about movie make-up, prosthetics, and masks: You can only add to the foundation, you can't take away what's already there.

You make terrain in an area up to 1 mile square look, sound, smell, and even feel like some other sort of terrain. 

For all intents and purposes the terrain is treated as real, which makes the Illusory Reality feature of Illusion Wizards a little redundant.

The terrain's general shape remains the same, however. Open fields or a road could be made to resemble a swamp, hill, crevasse, or some other difficult or impassable terrain. A pond can be made to seem like a grassy meadow, a precipice like a gentle slope, or a rock-strewn gully like a wide and smooth road.

You can't change the overall shape of the terrain. So no putting a tower, or even a building, in the middle of an otherwise open field. But if there's a large boulder in the middle of the field, then you can make it look like a building. But if anyone tries to get into the building it's permanently locked, or it opens to a cave-in, or just a brick wall because we can't take away from the boulder that was there. You can make a sparse forest-line more dense or appear as a wall, but you couldn't change a forest to look like a field. Nowhere does it say you can remove existing elements of the terrain. Instead it tells us we can...

Similarly, you can alter the appearance of structures, or add them where none are present. 

You can change what structures look like. I would define structure to not only include man-made objects like buildings, but also include trees, mountains, boulders, and other sufficiently large terrain pieces. And while you can add structures where none were present, we still have to follow the rule of not changing the overall shape of the terrain (think like the "Content Aware Fill" in Photoshop).

If you're an Illusion Wizard with Malleable Illusions, the parameters of this spell are implying that you can add to the terrain within reason, but can't take away from it. And Malleable Illusions says you need to act within the spells parameters. So you can't put a hole in a wall and make it real, but if you fill in a hole as part of this illusion then you can temporarily "remove" that part of the illusion to reveal the original hole and close it again after you're through.

 The spell doesn't disguise, conceal, or add creatures.

I would agree with those who said said this means you can't make a creature look different, you can't make creatures unable to be seen (invisible), and you can't add creatures. This is the one exception where we aren't allowed to add to the illusion, but it makes sense because creatures aren't terrain. I would say you can box in a creature to "hide" them assuming it's reasonably within the "maintaining the general shape of the terrain" parameters. There is also some DM adjudication here as to whether existing creatures would be suddenly stuck in a wall if you created one in their space, or pushed out into the nearest open space after you cast the spell which is how a lot of spells handle things like that.

The illusion includes audible, visual, tactile, and olfactory elements, so it can turn clear ground into difficult terrain (or vice versa) or otherwise impede movement through the area. Any piece of the illusory terrain (such as a rock or stick) that is removed from the spell's area disappears immediately.

Basically a repeat of the first sentence. The illusion is treated as real, but if a loose illusory piece of the terrain is picked up or removed then it disappears. This is the one place where an Illusion Wizard's Illusory Reality can come into play since you can make something like a rock or stick real and pick it up without it disappearing.

 Creatures with truesight can see through the illusion to the terrain's true form; however, all other elements of the illusion remain, so while the creature is aware of the illusion's presence, the creature can still physically interact with the illusion.

The illusion is treated as real even if the characters within know it's an illusion,  and can see through it. So if they see you covered a hole in a wall, they still can't pass through the hole. But a 7th Level Dispel Magic or Dispel Magic with a DC17 spellcasting ability check could remove the entire thing.

I don't know how much extra use an Illusion Wizard can get out of this spell since it already does a lot of the things Illusion Wizards get to do, but it does allow non-Illusion Wizards to pretend they're Illusion Wizards.

I've seen a lot of comments about the fact that it's not a cube limiting the height of the illusion.
I would argue that that the fact that it isn't a cube means it has no height limit (as a cube would make the height limit 1 mile). The spells' only limit skyward would then be based on the range of the casters' sight. The spell does says you can make structures. It doesn't say that you can't make structures that are taller than the examples in this spell.

A good rule of thumb with illusion magic in general is to conform to the limits, but beyond that anything's fair. 
You are already be able to make a mountain with this spell, why not also allow a tower as tall as that same mountain? And in that case, why not taller? 

Also for the first few comments about making parts of the spell real with illusory reality: there's no need! It's real for all intents and purposes already. It can deal damage, stop movement, and potentially kill or imprison creatures.
In fact, if you were to use illusory reality on an object in the illusion, you would be removing that object's ability to deal damage.

Can this spell make Fug?

This seems to me to be Hallucinatory Terrain on steroids.  Keep in mind this not an Enchantment, Conjuration, Evocation, or Transmutation spell. It is an ILLUSION spell. It is designed to fool those who interact with it. There are differences, yes, but they are enhancements to Hallucinatory Terrain.

Additionally, the ONLY tag involved is the Control tag. There are no damage or other tags associated with the spell, therefore, the spell itself can not do damage. Given its size, I suspect it is designed to control troop movements. IMO, the defining statement is 

Creatures with truesight can see through the illusion to the terrain's true form; however, all other elements of the illusion remain, so while the creature is aware of the illusion's presence, the creature can still physically interact with the illusion.

There is no save. If the above statement means what I think it does, whether or not a creature identifies the terrain as illusory doesn't matter. Creatures with Truesight MAY choose to interact with the illusion as if it were real, all other MUSTinteract with it as if it were real. This is a major enhancement.

The concealment issue is a problem.  All illusory terrain from any angle will conceal a creature from the opposing angle.  A bridge of earth over a crevasse that has a small stream with fish in it at the bottom technically conceals the fish.  The Adamantium tower conceals anything on the other side of the tower from you (and you from them) so you'd have a bunch of holes all throughout the illusion if that was the case.  I'd be willing to negate that aspect of that spell in most circumstances just to make it work at all.  Given that it is a separate sentence, I do not think the intent is that it cannot create things that conceal creatures, but that concealment of creatures is not an aspect of the spell.  You can't turn yourself into an illusory moving hill for instance. 

Assuming you listen to him, Crawford did explicitly say that Mirage Arcane can in fact cause damage. 

Crawford: The mirage arcane spell gives you tremendous latitude in how you make the affected terrain look and feel. The altered terrain can even hurt someone. You could drown in the spell’s illusory lake, for example, or fall off an illusory cliff. #DnD

Quote from MisterAkuru >>

"Similarly, you can alter the appearance of structures, or add them where none are present."

As a generic example, would a giant statue of a hand (maybe made from bricks) count as a structure?

Does Malleable Illusions animate the change of Illusions, or just switch over? For example, using the tactile nature of MA, slide buildings into each other, toppling a building over, or animating aforementioned statue to grab, punch or slam into enemies?

 Better. Malleable Illusions is insane for this spell and Hallucinatory Terrain because it makes those 10 minute casting times 1 ACTION, after you've set them up. My favorite thing to do is create an illusion that looks just like the actual terrain, and then once enemies enter, you begin making them question their sanity as you sculpt the environment every 6 seconds.

Jesus.

Basically, anyone you don't like is Leonardo Dicaprio and his team in Inception, and they are guests in your very lucid dream.

My level 19 tabaxi bard (play by post) has this spell for one thing and one thing only. Holidays.

thats cool interaction