This one seems potentially tricky to balance across all builds. Currently monsters can split by themselves and multiply to take over the level, and also multiply when you attack them in melee combat.
Monsters splitting by themselves doesn't seem like a big problem, although it would be nice if the newly arrived monsters were unable to act on the first turn they appear (I died at least once because of this, trying to fight through early level rat king room, rats multiplied and killed me same turn with no chance to react).
The bigger problem is monsters that call for reinforcements when you attack them in melee - they have a chance to do this *every time you attack*, not just once per turn. Even a character with a weapon that never misses (blacksmithing with diamonds, or some legendary weapons) can have trouble with that kind of monster. You can be a melee-focused build, and still be unable to get rid of them unless the build focuses on massive damage or very high crit chance - builds that focus on having high attack but average damage will still fare poorly against them.
This also means that you can end up killing a monster in one turn, while still being able to act (step up trait), and yet the monster was still able to call reinforcements.
I think a good balance change would either be to make the call reinforcements ability only be usable by a particular monster once per turn, or else have it only be used once *all* your attacks finish. If you kill the monster in one turn, if probably should not get the chance to bring friends. Or if called out for help, and no one came, calling out again the same turn shouldn't work either.
Balancing monsters that 'split' or call reinforcements
Re: Balancing monsters that 'split' or call reinforcements
For monsters that 'split', as opposed to the ones that call in friends, another idea is to have the remaining hp be split between the original (parent) and the newly split one (child). This would be fine to ignore any potential changes to how reinforcements work, and would serve to differentiate the two kinds of threats.
So the problems that melee builds still have with reinforcements would not necessarily be the same problems with goo blobs spitting. If blobs were to split on every single hit, but hp pool was also split then they can still wade through hacking and slashing and it will feel just fine.
Splitting the hp pool when the monsters split also make the regeneration that some blobs have a cooler thematic ability, maybe the normal way they multiply is splitting when at full hp, instead of spawning copies at full health next to themselves.
So the problems that melee builds still have with reinforcements would not necessarily be the same problems with goo blobs spitting. If blobs were to split on every single hit, but hp pool was also split then they can still wade through hacking and slashing and it will feel just fine.
Splitting the hp pool when the monsters split also make the regeneration that some blobs have a cooler thematic ability, maybe the normal way they multiply is splitting when at full hp, instead of spawning copies at full health next to themselves.
Re: Balancing monsters that 'split' or call reinforcements
It is also notable that even sleeping monsters that are missed can call reinforcements from the very first miss in a turn. Monsters sure do sleep lightly!
Re: Balancing monsters that 'split' or call reinforcements
These are two different abilities/mechanics you describe:
Backup
If the monster survives an attack, there is a chance it will call additional monsters that will surround the player.
Cloning
Monster has a chance to spawn a clone of itself in a nearby tile every turn.
I've also noticed the Backup mechanic can become annoying or tiresome (especially monsters who have both abilities!). But this is how it was in the original Lost Labyrinth so I've kept it so far. One way of solving this would make it so that monsters can only call for backup once - and then never again. Also monsters who were called for backup could be prevented from calling for more help.
This would make the game slightly easier (but more fun to play?).
Backup
If the monster survives an attack, there is a chance it will call additional monsters that will surround the player.
Cloning
Monster has a chance to spawn a clone of itself in a nearby tile every turn.
I've also noticed the Backup mechanic can become annoying or tiresome (especially monsters who have both abilities!). But this is how it was in the original Lost Labyrinth so I've kept it so far. One way of solving this would make it so that monsters can only call for backup once - and then never again. Also monsters who were called for backup could be prevented from calling for more help.
This would make the game slightly easier (but more fun to play?).
Re: Balancing monsters that 'split' or call reinforcements
I'm pretty sure I had the backup monsters not call for backup themselves. So monsters that are marked as spawned = no loot would also never trigger the backup trait.