I'm just experimenting with this mod to make sure it works and will fulfill my needs. In testing, I have turned off all mob spawns except for skeletons, and have attempted to multiply their health by 2. My spawn.json file is shown below:

https://gist.github.com/coryjreid/1ae9b2c5d58d613d4db3b613b9e10c47

Expected Result: All skeletons spawn with 20 hearts (40 health).
Actual Result: Skeletons spawn with a variety of health values from the expected to 512 hearts (1024 health).

Am I doing something wrong? I also tried using damagemultiply to reduce the strength of the skeletons (my ultimate goal) but it appeared to have zero effect. No errors were produced in the log via debug mode or otherwise.

Is this an incompatibility with the newer Forge version? Needed the newer version because of JEI and Morpheus.

Minecraft Version: 1.12.2
Forge Version: 14.23.5.2836
Mods Present: https://i.imgur.com/ULLTERb.png (please forgive my screenshot, I was too lazy to type it all out)

Having the same issue

[{
		"mob": ["minecraft:skeleton"],
		"nbt": {
			"HandDropChances": [ -327.67, -327.67 ]
		},
		"healthmultiply": 2,
		"result": "default"
}]

Skeletons have no double health or the correct HandDropChances

What does HandDropChances do?
Also what is the entire rule file?

My entire file is included in the linked Gist. All my other files are empty.

I personally have no idea what that NBT tag is.

Changes the drop chances for items in main-hand and off-hand (e.g. Bows for skellys)

Full spawn.json
https://gist.github.com/Insane96/660ae540242bb70e0c1535ae0df3ddfc

How are you actually spawning the zombies? The rules only fire for natural spawns

Yep, eggs spawn do not work. Natural spawns have doubled health but have no modified NBT

Ok, so maxhealth is working correctly then. The modified NBT problem is related to another issue which is that you cannot apparently modify all NBT with the method I'm using. For example, customName isn't supported either. I guess I'll have to provide custom actions for those NBT values that are not working. Can someone explain to me what those HandDropChances are exactly?

HandDropChances: List of float values representing the chance for a handheld item to drop.
ArmorDropChances: List of float values representing the chance for a worn armor item to drop.

I've been exploring this a bit more and wanted to update with my results.

History
I didn't specify but should have: all of this testing was occurring in single player. I was building a modpack at the time and so testing configurations locally and via LAN games to make sure things worked as desired.

Latest Mods In Use
I removed all mods except:

• In Control!
• The One Probe
• OptiFine

I discovered in testing that OptiFine is 100% the culprit (big surprise - not).

Steps to Reproduce

1. Setup In Control! spawn.json config file as follows:

[
    {
        "mob": [
            "minecraft:skeleton"
        ],
        "healthmultiply": 0.5,
        "result": "default"
    },
    {
        "result": "deny"
    }
]

2. Add OptiFine (1.12.2_HD_U_E4_pre4), In Control! (1.12-3.9.10), and The One Probe (1.12-1.4.28)
3. Launch the game and make a new world (creative, seed: 1)
4. Fly around checking skeleton health

With OptiFine installed: skeletons have between 0 and 10 health
Without OptiFine installed: skeletons always have 10 health (as desired)

I have since tested damagemultiply as well by setting its value to 0 in an effort to make it impossible for skeletons to damage the player. This does not appear to work as skeletons can still damage the player, but I wonder: is this possibly related to the fact that skeletons shoot arrows and the arrow is what actually damages the player? If so, is it possible to adjust the damage the arrows do but only to players (I assume probably not)? I ask this because if I multiply zombies' damage by 0, they are incapable of hurting me (as expected).

So in the end - I would say that the original issue that I reported seems not to be an issue afterall. And I wonder if you, @McJty, can shed some light on the possibility of adjusting arrow damage (as asked above)?

I also encountered the same problem in my modpack.

[{
    "result": "default",
    "mob": "minecraft:*",
    "healthmultiply": 3,
    "damagemultiply": 3
}]

https://github.com/Minecraft-Origin/Minecraft-Origin/tree/d948a5c5b65e4157ef1b8ebfa4c429d7bf621044

Add "onjoin": true to your rules. Damage and health options must be set in onjoin rules

Hey! I'm having this exact same issue. I removed optifine and added the "onjoin:" true rule, but its adding health 2 or 3 times on a single mob. If i spawn with an egg, it works 100% of the time. If the mobs spawn naturally, its almost always wrong at a 2-3 times rate.In the screenshot, I would expect 430HP (+400hp as my rule indicates, but its applied 3 times).

Spawn.json is here:
https://pastebin.com/K2YfWUHm

Example: