It's to build a fortress easily and adventure through it later, I think. Spawning stuff is impossible.
Actually there's a DFHack script collection which includes spawning creatures and normal DFHack commands allow for adding liquids and other stuff.
You could just copy the dwarf race and add flying to the creature. I have no idea about invisible, or if that exists. It's not to hard to modify how strong the creature should be if you give it like iron body parts or better, someone's certainly done it before because of the colossus gives you enough info to do it.
While spawning stuff is not impossible with dfhack(quite sure) but it could be done with workshops(custom made ones), maybe even create a new item which is equivalent to dungeon points/crystals, you'd take corpses to a workshop and it'd give you dungeon points/crystals which you'd spend at another workshop. Almost sounds like a cool mod.
I'm thinking a ghost type unit which covers flying and would prevent me from accidently killing things on the map I'm adjusting as they engage combat with my unit. I know the game has stealth which hides units, but ideally I would want invisibility so archers are not wasting arrows, bolts and other ranged attacks as I move thru the enemies which have better defenses. I can take control of a creature with DFHack, but I would need my ghost dungeon master to remain frozen and hidden until ready to leave.
There's a complete world editor coming (one day) that will let you do that.
While the game gets more and more procedural (with sliders) it will also get more and more moddable as people want control over their fantasy worlds. Along with that thinking, Toady mentioned an editor which he'll add so you can make fixed worlds, sites, myths, artifacts, etc. Perhaps not with Mythgen pass 1 (although Toady once said it was possible at it seems fun) but certainly later.
I think if Toady was going to live as long as elves the world editor would be completed -one day-, but based on what's planned and progress being made within the game... I have doubts we will see an editor. At this point Toady should be training two hardcore devoted developers which promise to continue the games evolution because if an unexpected tragedy removes him from this world then the evolution of the game ends(except for mods). Obviously these two developers would have to work for free because of love for the game... at least until Toady has passed the torch to them where they continue the same donation method being used today.
What's that? Does it, like, spawn stuff as if the player is on a quest and fulfilling objectives?
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I like to recreate towns and dungeons from old rpgs like Might and Magic. Is that what your aims are, to have some utility that slaps complete dungeons onto the map?
The dungeon master would modify a randomly generated world... such as:
A) making bridges between islands (perhaps bridges guarded by enemies).
B) balancing areas near the starting location so the player won't die during day one... and locations far away from the starting location provide greater challenges.
C) balancing the items, armor and weapons near the starting location so you don't start with iron armor before walking outside and providing better stuff at locations further away.
D) Using pressure plates to setup puzzles or types of traps.
E) Adding custom gods into the map using DFHack Script Collection.
F) Raising skill levels and placing armor on the main enemies in the map.
G) Placing trapped princesses on the map who need to be rescued
H) Setting up minecarts and tracks used as trains for traveling long distances
I) Creating an undead demon necromancer or dracolich
J) Creating ancient mysterious locations
K) and the list goes on and on and on.
Once finished it would provide a more interesting world for gamers. Ideally I would like to leave messages to help the adventurer, schedule future events such as major battles between races and custom quests but those don't seem possible with DFHack options.
