Artifacts version: Artifacts-1.16.4-2.7.5
Forge version: 35.1.37

Hostile mobs are not spawning naturally. Neither on the surface or underground.

The only exception is a superflat world, where hostile mobs do seem to spawn, or spawning hostile mobs using spawn eggs or through spawners. Ice & Fire mobs seem to not be impacted, nor passive mobs.

Removing this exact mod from a 182 mod modpack seems to fix the issue.

I don't have any logs on hand, but I can give you the modpack.
Here is the modpack import file for the CurseForge launcher:
(Note that Artifacts is included but disabled)

DynaStar Creates-Release Candidate 1.zip

Would you mind giving me a log file and your modlist? As far as I'm aware this isn't an issue when playing with just Artifacts, so there's not much I can do based on this information alone

@AceYacce would you mind giving me your full mod list? I've found this issue rather inconsistent to produce, but having your mod list as well might make it a lot easier to solve

@ochotonida I think he mentioned that he's running no other mods.

On another note:
I recently tested if this issue was still present in the pack while I was updating everything to their latest versions, and it still appears to be.

Hello
My Artifacts version is Artifacts-1.15.2-2.5.1
My forge version is forge-1.15.2-31.2.49
And my Curios API version is curios-FORGE-1.15.2-2.0.2.7*

The hostile mobs don't spawn for me neither, what is boring in survival.
And I don't have any other mods on this version of forge-1.15.2
That is

Thank you for your reading and attention, I sincerely hope that we can fix this problem in the future.

Thanks for pointing that out, I missed it when I read their comment. Though I just tested with the versions they specified, and mobs are spawning just fine for me, so there must be something they're doing different

In fact when I wrote my first message, in addition to having Artifacts-1.15.2-2.5.1, forge-1.15.2-31.2.49 and curios-FORGE-1.15.2-2.0.2.7 I used Hwyla-forge-1.10.8-B72_1.15.2, Bookshelf-1.15.2-5.6.40 and WAWLA-1.15.2-3.0.4

So, to be sure, I retested without any other mods (for real this time) except Artifacts-1.15.2-2.5.1, forge-1.15.2-31.2.49 and curios-FORGE-1.15.2-2.0.2.7 , but the aggressive mobs still do not appear (except Mimic) and I do not understand this bug.

Could it be from a different java version than yours or an incompatible forge version?

I was having this same problem in a custom 1.16.5 modpack I've been making for personal use, actually, and started doing some testing to figure it out. Artifacts 1.16.5-2.8.3, Curios 1.16.5-4.0.5.1, Forge 36.1.2.

From what I've seen, mimics apply to the hostile mob cap, and even with the default configs (and a 16 chunk render distance), there are usually over 50+ mimics in range at any time, especially if you use world gen mods that provide more campsite-friendly cave generation (I use Yung's Better Caves and Quark together, which creates fairly enormous, flatter areas underground prime for campsite generation, even with defaults, I've seen clusters of campsites 3-4 big in those places).

The moment I ran /kill @e[type=artifacts:mimic], other hostile mobs began to spawn immediately. Teleporting away to areas outside of that, hostile mobs stopped until I killed every mimic again. My last kill command on mimics recorded 138 entities killed (mimic chance was reduced to 20% from 30%, campsite chance upped to 16% from 8% in configs, with a 16 render distance)

Figured I'd swing by and mention that, if that helps any.

As an afterthought, I do also want to mention that there also seems to be a weird interaction with Team Abnormals mods. Removing those but leaving Artifacts in allows hostile mobs to begin spawning again as well, regardless of the number of mimics present.

Ayy, happy to help! Yeah, it just occurred to me when I went on an artifact hunt in one of my worlds and noticed that hostile mobs started spawning more and more the more mimics I killed in that area, so I did some kill command testing. Lo and behold...

You're entirely right, mimics count towards the hostile mob cap. I'll release a patch as soon as possible to fix this.
I already noticed myself that this only seems to become a problem under certain circumstances like you mention with Team Abnormals' mods, but I was never able to pin down the exact issue. (tbh I kinda dismissed mimics being a possible cause because they don't spawn like other mobs do, and I haven't made any major changes to them in a long time)