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Monster Behaviour Rules, Tarstuff and Element Interactions

DROD is fairly easy to pick up, and most of the monsters have movement patterns that become clear with a little observation and experimentation. However, sometimes things can get complicated (e.g. if there are brains in the room, or multiple wraithwings) and some puzzles depend on understanding the specifics of monster movement behaviour, so it's handy to have a guide to go into the details of how things work under the hood.

The descriptions on this page and in the individual monster pages are derived mainly from the excellent explanations by TFMurphy and bibelot on the DROD forum.

If you are already familiar with basic DROD monster movement and elements, we strongly recommend you play the Advanced Concept holds, downloadable from the holds page, which provide fantastic tutorials on the mechanics of monster behaviour and interactions.

General Principles
DROD is a deterministic, turn-based game. Moving the player character starts off the turn, with all monsters and other NPCs following this, in a defined order. For the most part, the exact order does not matter, but there are occasions where it might.

The order goes in groups as follows:

Within each group, movement order is according to the order they were placed in the room by the architect, followed by the order in which any spawned monsters (e.g. roaches laid by roach queens, babies formed from tarstuff) were spawned. In the case of mimics, they move in the order they were placed.

Following this, various game elements carry out their own behaviour, as discussed later.

Monsters have various types of behaviour, mainly differing in terms of which tiles they can move on, how they find their way to the player, how they behave when the direction they chose to travel in is blocked, what they consider obstacles, and how they behave when a brain is in the room. To see how monster behaviours differ, check out the individual monster pages.

Tarstuff is a unique element found in DROD. There are three varieties of tarstuff - tar, mud and gel - all obeying the same stability rules. Tarstuff cannot overlie or expand into pits, water, orthosquares, scrolls, arrows or any standard obstacle. Tarstuff cannot be less than 2 tiles wide, meaning that the smallest area tarstuff can cover is 2X2 tiles.

If a tile, or tiles of tarstuff are destroyed, leaving a one tile wide segment, this remaining segment is converted to tarstuff babies. Tarstuff can be destroyed by explosions, briar, removal by builders, ingestion by adder or by sword. Tar can only by cut on its edges, mud on external corners and gel on internal corners.

The intricacies are well covered in Martin Jacobson's excellent On the (Ab)use of tarstuff. Also see the individual pages on tar, mud and gel.

There are also tarstuff mothers, represented by a set of eyes imbedded in tarstuff. Tar and mud mothers affect any of their respective tarstuff in the room, whereas gel mothers only affect the gel blob they are connected to.

Every thirty turns, if the player is visible to a tarstuff mother a spawn will occur as follows. At the end of any spawning turn for the tar mother, any blank square (or trapdoor, platform, raft or open door) that is adjacent in one of the eight directions to a square of tarstuff will be filled with either that tarstuff or a tarstuff baby the following turn (unless a tarstuff baby would be formed under a sword, in which case it is immediately killed.) As follows from the stability rules above: tarstuff blobs will form in those squares in which it can be part of a 2x2 square of tar; all other tar squares will turn into tarstuff babies. The movement order for babies produced by tarstuff cutting is from left-to-right, row by row; whereas those produced in a spawn are formed in reverse order.

Tarstuff grows in the order: Tar, Mud, then Gel; but babies are only spawned after all stable tarstuff has finished growing for the turn.

Tar and mud babies move like roaches.
Gel babies move like rock golems and, like them, can walk in shallow water.
All tarstuff babies are invulvnerable to hot tiles.

Element Behavior Order
Here is the order that non-monster interactions take place and affect room elements:

  • Any non-flying monsters fall off platforms that have been moved out from under them, causing them to die

  • Tarstuff is destroyed through any sword stabs, allowing for simultaneous multi-stab removal

  • Unsupported Bridges fall

  • Fuses burn

  • Briar grows

  • Tarstuff grows

  • Pressure Plates that were depressed but no longer have weight on them now pop up again

  • Seep outside walls die

  • Green Doors toggle if the room is conquered

Just about everything else happens during the movement of whatever monster is currently being looked at:
Orbs are activated immediately when hit, Red and Black Doors activate as soon as the last trapdoor/tarstuff is removed,
Explosions occur immediately, with all connected explosions resolving before triggering any orbs they activate, and so forth.
Double placement is its own self-contained turn that happens after everything here. Once placed, the new Mimic/Decoy/Clone immediately interacts with the room (activates orbs and tokens, stabs tarstuff, etc).

Some miscellaneous element and monster interactions

  • Bomb and fegundo explosions activate orbs and destroy broken orbs

  • Items and monsters on closed doors are not affected by explosions. Explosions will not go against the direction of a force arrow

  • Mirrors block evil eye and Aumtlich gazes, but also reflect them

  • Mirrors and rock piles act as stepping stones when pushed into or formed in shallow water

  • Mirrors extinguish lit fuses

  • Briars can expand over water, orbs and mirrors, but not hot tiles or pits. They can expand diagonally over orthogonal squares

  • Briar growth, fuse burning and pressure plates popping up only occur on full turns (i.e. not during the additional half-turns the player gets when
    using a speed potion)

  • All monsters die when they spend more than one turn on a hot tile, with the exception of flying monsters and tarstuff babies

  • Platforms and rafts cannot be moved if weighed down by tarstuff, serpents or rock giants