Some of the AbyssalCraft mobs (some Lesser Shoggoth and Coralium Zombie are what I found, there may be others) spawn in the Twilight Forest. I don't think they should, and I also don't see anything in my AbyssalCraft config file (.minecraft\config\abyssalcraft.cfg, attached) to fix this conflict. Same with Twilight Forest's config file.

Versions:

• Minecraft 1.12.2
• AbyssalCraft 1.12-1.9.4-pre-4
• The Twilight Forest 3.1.128
• Biomes O'Plenty 7.0.1

If they're spawning there now, that means TF extends WorldProviderSurface, since no lair generation happened when this issue was reported (and the change for worldgen to be applied to any instance of WorldProviderSurface rather than Dim ID 0 wasn't long ago).

Sorry to necro this, but is there a way to disable the Shoggoth Lair worldgen in the TF Swamps while keeping them in the overworld?

Closing this now. Question has been answered.

1.8.6.1 is the only version of the mod that's ever had any Mob ID conflict (since I made the stupid decision to change the entity registration to use the global IDs instead of the dynamic I've used in all releases before and still use in all releases past that specific version).
The reason they're spawning is most likely that the one (or more) of the Twilight Forest biomes are registered in the Biome Dictionary as either WATER, BEACH or SWAMP (in this case it's probably the last one). Toggling "Lesser Shoggoth Overworld Spawning" will stop the Lesser Shoggoths from spawning, but the Abyssal Zombies and Depths Ghouls will still have a smallish chance of spawning.

I see. It's not a big deal really, it just seemed unintented behaviour at first glance, so I posted it. Thanks.