NCF – Extended list of Neutral creatures. Auto-translated

In my opinion, we can keep the immunity to water and mind control, give them a couple of spells, and an elemental ability. Perhaps regeneration as well. Everything else doesn't logically follow.

the problem is that magic in Heroes 5 is extremely weak. While fire/air can be somewhat compensated for with Wave of Fire and Chain Lightning, and Devil's Strike, the rest is completely useless. It's absurd that 1000 mages can deal only 700 damage with a Fireball. If you turn him into a warlock, he'll be weak.

But what's needed is a strong warrior, bearing the name of a water deity.

Yes, but the issue isn't about magic here. With that kind of ability logic, you could apply it to any unit. I suggest making it a highly offensive unit with decent casting capabilities. Defense: 25-35, below average. Attack: 55-60, good for rushing. Speed: 8, initiative: 13. Immune to mind control and water, double attack. Increase HP to 210. This would result in a powerful offensive unit with weak defense. A fearsome god, Neptune, vulnerable to fire, but explodes in a wave upon death.

Guys, are there any references to ancient gods in the lore?

Comrade Pastkha, you are right. We could tie it to some ancient "descendants" of water spirits...

And instead of calling it Neptune, call it (Little Neptune) – Senior Elemental, or Water Titans, or Titans of Water...

however, the suggested name is not Neptune, but just one of his warriors.

I would understand if some kind of spirit or spirit-spawn was very powerful (but not overpowered). The Warriors of Neptune (Poseidon) could be made into a creature of level 2-3, something like a warrior with a shield and trident. Give them the ability to cast a couple of spells, have high initiative, and good speed. Attacks above average, defense below average. HP above average by 4-5 units. Growth: 11 at level 2, down to 9 at level 3. What would we have then: a decent unit to obtain early in the game, capable of carrying the team with spells and fast attacks. Not vulnerable to kiting. In the later stages, it will perform well with its good attack and initiative, combined with decent growth and HP.

Not Neptune, but Shalassa, I agree with psatkha. But it doesn't sound as interesting anymore, and the connection to all sorts of tridents is unclear. You could try to diversify the game's lore and introduce some new, less common cult. Honestly, I was already getting tired of the focus on elemental dragons ten years ago, and Ubisoft made a mistake by making it so one-dimensional at the time. But still, it would be better if it wasn't Neptune, but someone or something else, not such an obvious copy of real pagan religions.

The Cult of Trabortall is based on the unexplored and rocky islands of the Jade Cove. Its members enlist the hidden powers of water spirits, which can become a good support in battle. In their manners, they resemble elves; it is not surprising that each Silad can be seen in elven ports; with their inherent agility, they are capable of inflicting a large number of accurate blows.

Level: 3.
Attack/Defense: 10/5.
Initiative: 12
Speed: 6.
Shots: 0.
Mana: 24.
Spells: Ice Bolt (enhanced).
Abilities: immune to water, living creature, small. Double attack, agility.

Trabortall Warrior

Perhaps this is what you meant?)

Yes, Trabortall, all sorts of Bormotoglots, that would all look like an interesting addition to the lore. Of course, for this to work, it would be ideal to have several mentions in different places to make it harmonious, but even so.

Tra-bor-talla: the power of water flows from the monolith!

Regarding cults and religion, there could be cults of powerful elemental spirits, like a father sky and mother earth! Or saints (Alaric needs to be canonized! He fought orcs in the name of humanity even when he was going crazy).
Regarding the fact that water doesn't burn: there was an incident in the Pirates of the Wild Seas expansion – evil nagas wanted to cause a mega-explosion using a crowd of fire elementals and a crowd of water elementals.
In general, elementals are most likely not made of matter, but of magic, so a magical fireball burns magical elementals.
In Might and Magic X, you could talk to elementals; they said that elementals, elements, and dragon gods are all connected. Falagar also talked about interesting things – that magic may not come from gods, but from mortal souls, and souls can be upgraded (yes, Seventh Dragon, we're talking about you).

It's a pity that in Heroes 5, elementals can't cast protection against their own element, like they could in Heroes 3 (and there were also mass spells for that). The Chaos magic branch would have expanded significantly.

found one instance of using the name of a Roman god:
Vestals - named after the Roman goddess Vesta, but in Heroes, Vesta is the widow of Emperor Ronan II. Most likely, the reference was to Rome, and the goddess was just an addition.