Hydroloths were hideous amphibians that resembled squat humanoid toads and stood ten feet tall when standing, although they were more often resting in a crouched stance. Their skin was warty, yellow, covered in mucus, and adorned with ancient, intricate symbols. They had frog-like faces that hosted a pair of bulbous eyes, webbed digits ending in claws on their hands and feet, and gliding flaps under their arms.
Hydroloths had transitioned from offers of power to those of information, trading items, and conjuring schemes to promote themselves through wealth and goods. Despite learning this, they still barely understood the true value of their various goods and were as likely to trade garbage for a fortune as they were to give away valuable artifacts for meager compensation. They approached the Blood War with great enthusiasm compared to other daemons.
Hydroloths were more dangerous in the air than on the ground, using their webbing to glide through the air like flying fish. While gliding, they could rip their foes apart with their foot claws in addition to their hand claws. Some purported hydroloths could spit a yellow, sleep-inducing poison produced from a gland at the back of their tongues. The odorous liquid put their enemies to sleep for a few dozen seconds, and only magic could awaken them if they succumbed to its effects. Because of their aquatic nature, they were resistant to water-based effects and harder to hurt when submerged within it.
Hydroloths could also attack with their claws and bite and could innately produce the effects of spells such as control water, crown of madness, darkness, detect magic, dispel magic, fear, invisibility, phantasmal killer, suggestion, and water walk. They were also capable of teleporting a limited distance at will through an effect similar to dimension door. Another hydrodaemon could be called to their aid half of the time. When in the presence of a large body of water on the Material Plane they could attempt to summon a powerful water elemental to assist them, although they could not magically control their summons. 
Like the river they inhabited, hydroloths also stole their victims' memories by merely focusing on them from a distance. A creature whose memories were lost to a hydroloth became a mere husk, incapable of even the most basic tasks unless the effect could be resisted, after getting several days of recovery, or upon the casting of a remove curse or greater restoration spell.
In terms of underwater offensives and amphibious missions, hydroloths were unmatched, a fact that earned them acclaim among the yugoloth community. They relished fighting the Blood War, as they drew strength from the horror of mortals. Weaker foes were gladly savaged while foes potentially capable of taking them on were avoided, although they took measures to stack battles in their favor and excelled at ambushes, covert operations, and guerrilla tactics. The crouched, toad-like position they normally adopted allowed them to spring from hiding at a moment's notice, often pouncing on their enemies in an attempt to knock them into a nearby water source. This tactic was at its most potent when within the memory-stealing River Styx, and as such few were willing to go against them with the Blood River nearby.
Because of their unparalleled skill in aquatic missions, hydroloths were awarded prestige they used to obtain wealth, mortal life force and influence among their kind. Although capable of forcing them to obey, greater yugoloths normally tried to appease hydroloths to secure their assistance without the use of outright dominance. They moved throughout the Styx in groups of 10-30, existing as mercenaries, scavengers, and guides using their ability to navigate the river to their advantage. Worthwhile magical items and maps of the Lower Planes could be obtained through trading with hydrodaemons as well as stolen memories, although their poor pricing skills could confuse matters. They were adept at finding lost objects and were accomplished underwater combatants, frequently hired to locate items that had sunk into the depths of the Styx and participate in raids of ships and coastal villages.
After learning that a decrease in power could confer unforeseen advantages, in their own case the ability to keep their memories within the Styx, hydroloths could enjoy a position free from the hatred of their piscoloth states. They genuinely enjoyed fighting the Blood War, but in order to advance had a new lesson to learn. Despite their new position of esteem and prestige they were still viewed as expendable soldiers by more powerful yugoloths, and could be thrown into the deadliest parts of the Blood War at a whim. Once this realization was had, most desperately tried to advance into yagnoloths to avoid being sent into the brunt of the Blood War.
Along with, amnizus, and merrenoloths, hydroloths were among the few creatures known to be immune to the deleterious effects of the River Styx. They had a preference for residing in liquids such as water, fire, and lava, but could be found dwelling anywhere. Their body parts could be used as material components of numerous spells and potions.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Allen Varney, ed. (June 1994). Planescape Monstrous Compendium Appendix. (TSR, Inc.), p. 122. ISBN 978-1560768623.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Mike Mearls, Jeremy Crawford (May 29, 2018). Mordenkainen's Tome of Foes. Edited by Kim Mohan, Michele Carter. (Wizards of the Coast), p. 249. ISBN 978-0786966240.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Mike Mearls, Brian R. James, Steve Townshend (July 2010). Demonomicon. Edited by Scott Fitzgerald Gray. (Wizards of the Coast), p. 111. ISBN 978-0786954926.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Gary Gygax (August 1983). Monster Manual II 1st edition. (TSR, Inc), p. 30. ISBN 0-88038-031-4.
- ↑ J. Paul LaFountain (1991). Monstrous Compendium: Outer Planes Appendix. Edited by Timothy B. Brown. (TSR, Inc.), p. 5. ISBN 1-56076-055-9.
- ↑ Colin McComb (September 1997). Faces of Evil: The Fiends. Edited by Ray Vallese. (Wizards of the Coast), p. 70. ISBN 0-7869-3430-1.
- ↑ Colin McComb (December 1995). “Liber Malevolentiae”. In Michele Carter ed. Planes of Conflict (TSR, Inc.), p. 30. ISBN 0-7869-0309-0.
- ↑ Mike Mearls, Jeremy Crawford (May 29, 2018). Mordenkainen's Tome of Foes. Edited by Kim Mohan, Michele Carter. (Wizards of the Coast), p. 6. ISBN 978-0786966240.