Undeads
Most are corpses that have been revived using necromancy and can thus only move within the reaches of that magick. There are also some examples of corpses buried in land with high magick concentrations coming to life spontaneously.Their appearance is essentially human, but the undead have no memory of their former lives and very low intelligence. They usually form hordes of five to ten individuals, but there is no single undead that could be called the leader of these groups and they do not act so human as to consciously cooperate.
Another characteristic trait of the undead is their simplicity, which makes them particularly tractable. They have destructive tendencies and any creature that is their equal or inferior will be seen as prey and hunted down—humans and animals included.

Skeleton Sorcerers are more dangerous and durable versions of Skeleton Mages. They cast powerful elemental spells, but are vulnerable to melee strikes. They can cast an unholy form of anodyne, with a red sigil that heals the undead. Blunt weapons do extra damage to their bones.
Skeleton Sorcerers wear a decayed version of their wizard's  clothing - some of the color of the original can still be discerned. They seem to be less stupid than undeads but are extremely fragile to physical attacks to the point that even a well given fist may destroy those decaying bones.

Wargs
They look like a variation of Wolves, with blue-ish grey fur and black markings. Just like wolves, they often appear in packs, Their bite can inflict Poison and even if they don't take a lot of damage to kill, they  are very agile and often dodge attacks directed at them. Wargs leave a great amount of  rotten corpses when they kill and when killed, leaving the area quickly is suggested unless one does not want to meet bigger monsters.

The Maneater is a yellow worm-like creature resembling the tentacles of a Gazer. They are dangerous foes that spring out from random chests to eat anyone that dares open the lid to its trap. Maneaters hide in and spring from chests found in Nerabyss Isle. When a chest containing a maneater is opened, the opener will likely be grabbed and then attacked.
Maneaters have sharp canine-like teeth that fill their mouth and are perfectly capable of consuming people whole. The creature's skin has strange eye-like markings, possibly to confuse its prey or simply make it seem more threatening to other denizens of the Isle. The creature's physical attacks include a circular sweep attack in close quarters, and will attempt to shake any grappling character off. If in danger, they will leave a sigil on the floor, with a timer. If the timer reaches 0 before the Maneater has been defeated, death may occur to those inside said sigil.

Elder Ogre
Though the name may imply a weaker or more fragile Ogre, this is not the case. The Elder Ogres are more powerful than regular Ogres.
Unlike their younger cousins, Elder Ogres are attracted to males instead of females. Male adventures and pals should be wary.
As a carrion-eater, Elder Ogres are attracted to the presence of corpses and thus can appear randomly after or during certain battles in which dead bodies are present. Having a Male in the party gets the Elder Ogre excited, making its cheeks and buttocks flush red. It then targets and chases that Male without paying any attention to other party members. Of course, Ogres' loves consist into beating someone to death.

Bosses

Gorecyclope
Gorecyclope  is a gigantic enemy boss chained to a wall that, upon receiving some damage  or its helmet is removed, will free itself and will pick up a giant spiked club to engage the Arisen and party. A Condemned Gorecyclops shares similar features to a regular Gorecyclope, but it is significantly taller, with a longer reach and its attacks are much more damaging. Like its brethren, it is weak against Ice-based attacks.
This particular enemy can be very difficult and time consuming as it has both high health and defense. Be very wary when casting spells as it will frequently rampage.

The Eliminator
The Eliminator is a huge humanoid resembling a minotaur. It stands taller than the Arisen, though smaller than an ogre, and is scalable. It carries a massive hammer as a weapon. It was once an executioner that got corrupted by the dark in the Isle and lots its humanity.
The Eliminator's attacks consist of many large-radius hammer blows, and just like an ogre, they are much faster than one would expect. Eliminators don masks, Fur Boots and  Belts, and are armed with a huge hammers. There are two variants, either light or dark skinned, one has downward curling horns rather than upward. When a someone is knocked to the ground, the Eliminator may place a foot on that person, thus pinning  them to the ground. The Eliminator charges a very powerful downward strike which can prove fatal unless interrupted or the character successfully evades.

Part 2
Banshee
They appear as an ordinary-looking undead woman, however, they have some fearful abilities that can become dangerous if there are a group of them. Banshee unleash a paralyzing shriek which can stun everyone within range, allowing other foes to gang up on the helpless. They are not weak against holy magic but they are terribly weak to fire and ice elemental spells. If one is brave enough, they could simply grab them and toss them into a flame or in a cliff...

The Cockatrice is a large, rooster-like monster that has the body of a ferocious beast and the wings, legs and head of a rooster. While sharing similar features to that of a Griffin, Cockatrices have jet black, scruffy feathers that add to its nightmarish appearance and wild nature, as opposed to the majestic mien of a Griffin. The Cockatrice has a large throat pouch which stores its deadly petrifying gas, which when released, can turn any living being in its wake into solid stone and suffocate them to death. Its rotten corpse may attract other monsters.

Greater Goblins and Goblin Shamans
The former ones, as the name implies,  are larger and more fearsome than their lesser goblin counterpart. They charge into battle, wielding large maces and attacking with a berserk fury that makes them a threat to even the most wary adventurer.
The latter ones  look like standard goblins except they have the ability to cast magic. The Goblin Shaman bears a staff once used by an apparent human mage. Goblin Shamans are the size of a small grimgoblin or normal goblin.  They can cast magic and are heavily resistant to it. However, they have little defense against physical damage. Just like Wargs, their rotting corpses will attract other monsters.

Garm
Garm are huge, wolf-like quadrupedal beasts that typically appear in packs.  Prior to noticing the someone or party members, Garm will be rending and eating the corpses of the slain.
Garm are necrophagous creatures; the stench of death and rotting corpses attracts them. Their appearances are not entirely random as the adventurers can influence the spawn rate of Garm.  Garm belong to the Wolf family of monsters. Garm can be encountered in packs of 2-3, or as solitary individuals.

Fallen Dragons
Fallen Dragons come from origins unknown and are in an advanced rotting phase most of the times. Those are big, rotten fallen drakes and dragons which no longer bear the intelligence nor the power of their living counterpart. They are attracted by life, and they will seek life indeed, trying to cling towards the only thing they remember.
Fallen Dragons are undead dragonkin, brought back to life by foul magic which takes the form of the crystal core in their chest, their weak spot. Instead of breathing fire, they breath a vile and foul gas, which rots any time-sensitive items and harms people by contact alone. They still have sharp fangs and claws, can hardly fly and are very weak to the element they once breathed with pride: fire.

Bosses

Gazer
Gazers are large floating eyeballs with protective "eyelids" that have teeth instead of eyelashes and are covered entirely in sentient tentacles that aid in both defense and offense.  The Gazer can also summon Tentacles from the ground which can each independently cast fireballs, ice shards or an electrical attack against the party.It Casts a powerful debilitating beam/gaze from its eye that can cause Torpor, Petrification, Poison, Silence and Blindness.
The debilitating gaze may be avoided by hiding behind cover, by turning away, or blocking it with a shield. When battling on the bottom floor, the Gazer uses additional attacks such as slamming against the ground, and spinning wildly. The Gazer can also fill the bottom floor with poisonous gas which will continuously damage any exposed party members standing on the ground floor. Seek high ground to avoid exposure. Summons both regular and large tentacles for a single, upward thrust. It can deal massive damage from the strike itself and additional fall damage on any characters standing above the tentacle thrust when it occurs. It chants spells using its eyelids like a mouth. Slicing the tentacles will just leave it only the gaze and spells to attack.

Hell Bishop
Once an arch-priest of a holy order, the Dark Bishop was twisted by the horrors of Nerabyss Isle into a corrupted aberration of his former self. The Dark Bishop is a powerful spell caster, wrapped in a protective sacred aura, capable of a magnitude of spells and various forms of incantations to obliterate unwary adventurers. It summons a Fallen Dragon as an ally and can manipulate the dragon to do its will. The Hell Bishop is engulfed in a Sacred Aura, greatly increasing its resistance to Holy-based attacks but also decreasing its resistance to Dark-based attacks. Persists through Possession of the Dragon where the  Bishop possesses the Cursed Dragon, able to cast stronger spells over the entire field. If the Dark Bishop is knocked out of possession (either through enough damage or if the possession target is killed), it will stun/fatigue the being, allowing adventurers to strike. It is greatly capable of using elemental magic, from basic to strong spells and just like Maneaters, when about to be defeated, it can invoke a magickal sigil capable of destroying outright any foe held within its span when it completes.

Leroy
Leroy was a Hero among Heroes from an age long past, who refused the choice given to him by the "goddess" of Justice Veritas. In return, the deity transformed him into this foul creature gone mad with despair at the loss of his lover. Found in the sanctuary.

It is believed this evil entity was born out of the past hero's hatred towards the destiny imposed upon him. This monster nurtured from these ill thoughts for centuries, feeding itself from its host as well as the unfortunate adventurers who met their doom in this dungeon. Leroy's true strength is unknown.
Given the amount of corrupted thoughts it has consumed, it could be depicted as the embodiment of mankind's chaos; humanity's desire to end the infinite cycle. Despite his size and bulk, Leroy is a powerful and agile fighter with potent spell casting and brute force. He will use tactics to lure the party into disarray, which makes him a tough opponent. He is generally highly resilient to both physical and magical attacks; if one intends to deal noteworthy damage, they should target his head. Halfway through battle, Leroy will begin to alter his rhythm, casting deadly combinations of magic that may deal serious damage to fighters. He performs his special move more than once during battle, and it can drastically change the course of the fight: Void Vortex where he quickly charges and slams the ground creating a vortex, slowly drawing in nearby targets. If the attack is fully completed, targets caught in it will be instantly killed.  Interrupting the attack prematurely will cause him to be stunned for about 20 seconds and take increased damage.