This is just a quick post as to how "monsters" are handled in OSR By Night.
The creatures of your game world can be divided into two broad categories:
"The Normal" and "The Others".
The Normal is basically the mass of humanity, animals and so on.
These follow all the normal rules you'd expect in an OSR game.
Notably they obey the following rules:
They are subject to morale scores.
They roll 1D6 per level for hit points.
They never inflict panic and sanity checks.
The Others is anything that does not exist in the real world: Undead, ancient monsters, lovecraftian horrors, alien infiltrators and so forth.
They obey the following rules:
They do not have morale scores, though they may be subject to other factors influencing their behaviour.
They roll 1D8 per level for hit points.
They are treated as "large" creatures for unarmed combat purposes.
They may inflict panic and sanity checks if their hit dice exceeds the experience level of the characters.
I ran into an elder god. Now what?
When first encountering "the others" unexpectedly, there is a chance the creature will basically ignore the puny humans, unless they draw attention to themselves.
This only occurs if the Others have more hit dice than the highest experience level. Roll 1D12. If the score is equal or under the difference in hit dice/levels, the monster will simply proceed with whatever it was doing (look for worthier prey, build the gateway to its home dimension or trample cities).
This of course assumes the characters don't draw attention to themselves and seek some sort of cover to hide behind.