Bog Fiends

Aquatic: This creature can breathe underwater indefinitely.

Demonic Form: This creature is immune to spells or spell effects that change their form unless a greater demon is the caster. Additionally they're immune to the Charmed, Frightened and Poisoned condition.

Plague Demon: This creature is immune to Acid, Necrotic and Poison damage.

Multi Attack: This creature can make two Tentacle attacks or one Acid Stream attack each round.

Repulsive Smell: Any creature within 30ft of this creature automatically knows the location of this creature by its stench. If there are multiple Bog Fiends in radius of each other, the nearest one is detected above all others.

Slug Foot: This creature can traverse sheer surfaces without using their arms. Additionally, they have advantage on saving throws to resist being pushed.

Unnatural Being: This creature doesn't need to Breathe, Drink or Sleep. Additionally they have resistance to non-magical Bludgeoning, Piercing and Slashing damage.

Acid Stream: 5ft/10ft range, DC14 CON save. A creature hit by this attack takes 4D8 Acid damage (half on a success rounded up).

Tentacle: 5ft/10ft range, +7 to hit, 1D10+4 Bludgeoning or Piercing damage. If this attack hits and deals Piercing damage, the target must pass a contested STR save or become Grappled.