Enemies

Author(s): benevolusgoat
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Crash course table of contents

The crash course is still a work in progress!

1. Creating a mod.
   1. Creating the mod.
   2. Uploading a mod.
2. Creating a passive item.
3. Creating an active item.
4. Adding to item pools.
5. Making costumes.
6. Making a new character.
7. Creating entities.
   1. Entity basics.
   2. Creating effects.
   3. Creating enemies.
   4. Creating a familiar.
8. Adding sounds.
9. Adding music.
10. Making pocket items.
    1. Creating custom cards, runes, and objects.
    2. Creating custom pills.
11. Creating challenges.
12. Structuring your mod.
13. Conclusion.

Enemies (or EntityNPCs as the game defines them) make up an important part of Isaac's gameplay. This tutorial will cover the bare essentials for creating an enemy. This tutorial will not cover enemy AI or having them appear within rooms.

Video tutorial

entities2.xml entry

As with any entity, the enemy needs to be defined first. The previous article on entity basics covered many of the tag variables that could be applied to any entity. This section will cover tags that have exclusive functionality to enemies.

entity tag variables

Variable Name	Possible Values	Description
boss	int	Whether or not the entity is considered a boss. 1 = allowed, 0 = disallow. Default = '0'.
bossID	int	Custom entities cannot take advantage of this. The unique boss ID associated with the entity, used for other things such as entries inside bosspools.xml.
champion	int	Allow champion variants of this entity. 1 = allowed, 0 = disallow. Default = '0'.
stageHP	int	How much additional maximum health the entity gains with each stage. Read here for more information.
portrait	int	Used in conjunction with the entities deathanm2 tag to determine what portrait to show on Isaac's Last Will. The number correlates to the frame in the anm2 file, starting from 0. Required for the entity being displayed in the Bestiary. See Addiing the enemy to the Bestiary for more information.
hasFloorAlts	bool	If set to true, floor specific sprites should be used for this entity if they exist. Default = false. See Floor specific sprites for more information.
reroll	bool	Deprecated as of Repentance. Use the noreroll tag instead.
shutdoors	bool	Determines whether this entity will prevent doors from staying closed. Only used for enemies. Default = true.
shieldStrength	int	Entity takes less damage relative to Isaac's DPS, known primarily as "armor". The higher the number, the less damage is taken. Default = 0. See this page for more information on the mechanic.
bestiaryAnim	string	Unused by vanilla and was an old way for defining bestiary entries. See Addiing the enemy to the Bestiary for more information about the Bestiary.
bestiaryOverlay	string	Unused by vanilla and was an old way for defining bestiary entries. See Addiing the enemy to the Bestiary for more information about the Bestiary.

A full entities2.xml entry on a custom enemy named "Level 2 Roundy". Refer back to this entry for subsequent chapters below.

<entities anm2root="gfx/" version="5" deathanm2="gfx/enemy_death_portraits.anm2">
    <entity name="Level 2 Roundy"
    id="678" anm2path="monsters/roundy2/roundy2.anm2" portrait="0"
    collisionMass="100" collisionRadius="20" friction="1"
    numGridCollisionPoints="8" shadowSize="0" champion="1"
    baseHP="22" stageHP="0" collisionDamage="1">
        <gibs amount="6" blood="1" eye="1" gut="1" />
        <bestiary anim="Idle" transform="0,0,1" />
        <devolve id="276.0.0" weight="1" />
    </entity>
</entities>

Defining IDs for custom enemies

There are hundreds of mods on the Steam Workshop that add custom enemies. Unfortunately, if two enemies from two different mods happen to have the same id and variant, it will not automatically assign a new variant.

To mitigate this conflict occuring, group your enemies under the same id and make lots of variants for completely custom enemies, or repeat this for variant and subtype for variations of existing vanilla enemies. Define arbitrary large numbers under the cap of 4095 (or 255 for subtypes) to lessen the chance of conflicting IDs.

Floor specific sprites

If an entity has the attribute hasFloorAlts set to true, the game tries to load the spritesheet of the entity with an additional suffix, based on the current stage. The suffix of a stage is defined in the suffix attribute in the stages.xml file. If no sprite can be found, it will load the default spritesheet.

For example, if the original spritesheet's file path is monster_017_gaper.png and the suffix is _downpour, the spritesheet will be replaced with monster_017_gaper_downpour.png if it exists.

Suffix per stage:

Stage-Name	Suffix
Flooded Caves	_downpour
Downpour	_downpour
Dross	_dross
Ashpit	_ashpit
Mausoleum	_mausoleum
Gehenna	_gehenna

REPENTOGON adds a floor suffix to every stage in the game.

REPENTOGON stage suffixes

• Flooded Caves does not have its own suffix due to the vanilla game tying it to Downpour instead.
• The Void does not have its own suffix as it generates random rooms from other stages and uses the local stage ID of the current room for its set of floor alt skins.

Stage-Name	Suffix
Basement	_basement
Cellar	_cellar
Burning Basement	_burningbasement
Caves	_caves
Catacombs	_catacombs
Flooded Caves	_downpour
Depths	_depths
Necropolis	_necropolis
Dank Depths	_dankdepths
Womb	_womb
Utero	_utero
Scarred Womb	_scarredwomb
Blue Womb/???	_bluewomb
Sheol	_sheol
Cathedral	_cathedral
Dark Room	_darkroom
Chest	_chest
Downpour	_downpour
Dross	_dross
Mines	_mines
Ashpit	_ashpit
Mausoleum	_mausoleum
Gehenna	_gehenna
Corpse	_corpse
Home	_home

devolve child tag

When this entity is devolved through the D10 collectible or Game:DevolveEnemy, this tag is used to determine what entity it should be devolved into. The game only uses the first devolve tag defined.

Variable Name	Possible Values	Description
id	string	The entity to devolve into. This variable is unique where the ID, variant, and subtype are all combined into one string separated by periods in the format of Type.Variant.Subtype (e.g. A regular Gaper would be defined as 10.1.0). Effect entities should use the type of 999 instead of 1000, as 1000 is reserved for rocks (used by Stone Grimaces). For rocks, the variant and subtype is ignored.
weight	float	Completely ignored by the game, but would likely be used to determine the relative likelihood of this devolution be chosen. Default = 1.

Level 2 Roundy has the following definition for their devolve tag, which devolves into a Roundy:

<devolve id="276.0.0" weight="1" />

Adding the enemy to the Bestiary

For your enemy to be seen within the Bestiary, three requirements must be met:

1. The bestiary child tag exists.
2. The portrait variable exists.
3. Your enemy has been encountered at least once within a run.

The bestiary child tag has a few variables to specify how the enemy is displayed in the Bestiary when selected. Any animations that play will automatically loop, even if not defined as a looping animation in the anm2 file.

Variable Name	Possible Values	Description
anim	string	Animation to play for the entity in the entity's anm2.
transform	string	A comma-separated string of X and Y coordinate position offsets and custom scale, defined as "X,Y,Scale". Scale is a multiplier. Default = 0,0,1.
overlay	string	An optional overlay animation to play for the entity. This will play a separate animation that renders over the anim sprite animation.
anm2path	string	An optional separate anm2 file to use exclusively for the Bestiary.
alt	string	An optional floor alt to use (e.g. _downpour). Omit to not use a floor specific sprite.

For the portrait variable, you will need an anm2 file defined in the deathanm2 variable within the entities2 root tag and placed within your mod's content/gfx/ folder. There must be one animation and one layer that contains frames for every single portrait. The first frame is frame 0, so your portrait variable should start from 0.

Level 2 Roundy has their portrait set to 0, alongside the following definition for their bestiary tag:

<bestiary anim="Idle" transform="0,0,1" />

The preview animation will play the animation in the anm2 file named "Idle". The X and Y offsets for the animation are 0, and the scale of the animation is 1.

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