What is it ?

This is a 1.12 mod which adds a couple of spooky creatures which are pretty simple in design and concept.

What does it offer ?

A couple of mobs , since there is not a whole lot I'll give more details down here (spoilers for the mobs added) :

Will-O'-Wisp :
An ambient defensive mob for Swamps (biome Type : SWAMP) which will burn you if you stand in its place; it is quite lazy in looks as its just a ball of colored light

Pumpkin Slime :
A weird slime which looks exactly like a pumpkin with various possible carved faces (it can be tamed). it sort of disappears during the day (if untamed), sometimes leaving behind an actual jack O' lantern with the same carved face as the original creature.
It spawns in biomes with the MAGICAL, FOREST or PLAINS biome Type

Nether Knight :
A what-if scenario on the Mob D from the first ever mob vote, this can be considered a miniboss for the Nether, which mainly attacks in melee. It can also randomly set its attacker on fire for a brief second after getting hit, or summon wither skeletons/blazes with higher hp stats (can be tweaked in configs tho)
Killing it gives you a shield which can set attackers on fire

Cursed Armor :
A possessed armor accompanied by its sword, this creature works well when paired with Primitive mobs's Haunted Tool mob; It has only 1 hp, but you'll have to 'break' its armor before (which is easier to do when spam-clicking as it doesn't take into account how much damage you're dealing per hit, which reminds of the Absorber's gimmick from the mod Illage and Spillage)
It spawns in biomes with the WASTELAND, SPOOKY, DEAD or MAGICAL biome Types including biomes such as the roofed forest

I have not noticed any major issue with this mod or any problems with performances for that matter, but I've mostly underused these creatures when having the mod in custom modpacks so I can't really be sure that nothing bad could happen.

In conclusion

This is a pretty nice mod, though the mobs' design is pretty basic overall and their AI doesn't go that far .

Eerie Entities also has its own config files as well, which allows to tweak a bunch of stuff such as mob spawning and base stats.

If you are looking for more 1.12 mods adding useful entities such as these I recommend Fish's Undead Rising , Beast Slayer and Primitive Mobs (this one is discontinued, use alternatives like Primitive mobs Wondersapped instead) , though more might be out there.