I’ve made a few creatures to be used in the Under Dark for South Reaches. These are creatures that I created for The Fractured Mountains originally. The ones here are the Ankyloslime and the Strange Salamander. These have not been playtested so the listed CR may be incorrect. The links lead to their D&D Beyond page.
Ankyloslime are around 5 feet tall and appear to be dripping. Other than that they appear to be small Ankylosauruses. What gave rise to these creatures is unknown, but they can be deadly.
Medium Ooze, unaligned
13 (Natural Armor)
STR DEX CON INT WIS CHA
15(+2) 11(+0) 15(+2) 2(-4) 12(+1) 5(-3)
Darkvision 60ft., Passive Perception 11
Melee Weapon Attack: +5 to hit, one target. Hit: 14 (3d6+3) piercing damage.
Melee Weapon Attack: +5 to hit, one target. Hit: 14 (2d10+2) bludgeoning damage. If the tail is holding a rock and the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.
Ranged Weapon Attack: +2 to hit, reach 10 ft., one target. Hit: 12 (2d10 ) bludgeoning damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone. This attack cannot be made again until the Ankyloslime finds a suitable rock to be thrown.
The Strange Salamanders are seen as the race that the Salamen came from. These creatures are very large versions of the common amphibious salamander but have six legs and a large growth on their tails used to bash opponents. They can be just about any color and have lighter highlights.
Large Elemental, neutral evil
17 (Natural Armor)
60ft., burrow 40ft., climb 40ft.
STR DEX CON INT WIS CHA
20(+5) 12(+1) 18(+4) 12(+1) 11(+0) 9(-1)
CON +8, WIS +4
Darkvision 60ft., Tremorsense 60ft., Passive Perception 14
The salamander makes six attacks: four with its claws, one with its tail, and one with its bite.
Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 8 (1d6 + 5) piercing damage.
Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage plus 7 (2d6) fire damage, and the target is grappled (escape DC 14). Until this grapple ends, the target is restrained, the salamander can automatically hit the target with its tail, and the salamander can’t make tail attacks against other targets.
Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 9 (1d8 + 5) piercing damage and 5 (1d10) cold damage.
Acid Breath (recharge 5-6)
The salamander exhales acid in a 60-foot line. Each creature in that area must make a DC 17 Dexterity saving throw, taking 44 (8d10) acid damage on a failed save, or half as much damage on a successful one.