Deep one
Deep ones are new monsters added in the Lethe patch, SLASH'EM and dNetHack. There is a large level difference between the deep ones and deepest ones, so once you are experienced enough for deep ones to appear you will likely encounter them in some form until the end game. This also means that if a lower level character manages to tame a deep one (which is not difficult as their MR is only 10), they will eventually have a level 45 pet deepest one. All deep ones are capable of treachery, but will eat corpses and thus are relatively easy to keep tame.
The deep one is the only form that is capable of wearing body armor and cloaks, and all forms can wear all other types of armor. If a deep one wearing body armor grows up into a deeper one, the body armor is not destroyed as with polymorphing into a larger size, so it is possible to raise a deepest one that wears a full ascension kit of armor.
Deep ones are capable of swimming and will also be frequently generated on the Plane of Water.
Deep(er, est) ones play a much greater role in dNethack. They are abundantly generated on the Anachrononaut quest, complete with firearms and other future weapons. They are also generated in the the Gulf of N'Kai in the Neutral Quest. In addition, deep and deeper ones can be spawned in a dNethack throne room, with a deepest one as the overlord.
In addition, they each have a passive on-death effect. When a deep one dies, every other deep(er, est) one on the level receives +2 max hp. Deeper ones give +4, and deepest ones give +8. If this is enough to bring them to a new level threshold, they'll grow up into the next stage of evolution and deal more damage. This only applies to monsters currently on the level though, so it won't apply to monsters generated in the future.
Deep one
h deep one 
| |
|---|---|
| Difficulty | 9 |
| Attacks |
Claw 1d6, Claw 1d6, Bite 2d4 |
| Base level | 7 |
| Base experience | 92 |
| Speed | 9 |
| Base AC | 5 |
| Base MR | 10 |
| Alignment | −5 (chaotic) |
| Frequency (by normal means) | 2 (Quite rare) |
| Genocidable | Yes |
| Weight | 1500 |
| Nutritional value | 400 |
| Size | Medium |
| Resistances | cold, poison |
| Resistances conveyed | poison (47%) |
|
A deep one:
| |
| Reference | SLASH'EM_0.0.7E7F2/monst.c#line807 |
In dNetHack, they are slightly changed, with a 1d6 weapon attack, a 1d6 offhand weapon attack, and a 2d4 kick.
Deeper one
h deeper one 
| |
|---|---|
| Difficulty | 18 |
| Attacks |
Claw 3d4, Claw 3d4, Bite 4d6 |
| Base level | 15 |
| Base experience | 388 |
| Speed | 12 |
| Base AC | 0 |
| Base MR | 30 |
| Alignment | −7 (chaotic) |
| Frequency (by normal means) | 2 (Quite rare) |
| Genocidable | Yes |
| Weight | 2000 |
| Nutritional value | 500 |
| Size | Large |
| Resistances | cold, poison |
| Resistances conveyed | poison (100%) |
|
A deeper one:
| |
| Reference | SLASH'EM_0.0.7E7F2/monst.c#line834 |
In dNetHack, they are slightly changed, with a 3d4 weapon attack, a 3d4 offhand weapon attack, and a 4d6 kick.
Deepest one
h deepest one 
| |
|---|---|
| Difficulty | 33 |
| Attacks |
Claw 3d6, Claw 3d6, Bite 5d6 |
| Base level | 30 |
| Base experience | 1188 |
| Speed | 15 |
| Base AC | −5 |
| Base MR | 70 |
| Alignment | −9 (chaotic) |
| Frequency (by normal means) | 1 (Very rare) |
| Genocidable | Yes |
| Weight | 3000 |
| Nutritional value | 500 |
| Size | Huge |
| Resistances | cold, poison |
| Resistances conveyed | poison (100%) |
|
A deepest one:
| |
| Reference | SLASH'EM_0.0.7E7F2/monst.c#line843 |
In dNetHack, they are mildly buffed, with a 3d8 weapon attack and 3d8 offhand weapon attack instead of 3d6 claws. The 5d6 kick is the same.
Mythology
The deep one and its related kin all come from "The Shadow over Innsmouth" by H.P. Lovecraft, creator of the Cthulhu mythos.
They are intelligent ocean-dwelling creatures, and are fishy/froggy humanoids in appearance. The transition from deep one to deepest one in-game is a reference to the deep one-like giants in Lovecraft's earlier short story "Dagon." Deep ones are immortal, and in the wider Cthulhu mythos it is accepted that they grow continuously throughout their lives. The oldest deep ones, therefore, would be very large indeed, thus explaining both the human-sized deep ones in "Innsmouth" and the whale-sized giants living in the deep ocean in "Dagon."
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Encyclopedia Entry
For eighty thousand years Pth'thya-l'yi had lived in Y'ha-nthlei, and thither she had gone back after Obed Marsh was dead. Y'ha-nthlei was not destroyed when the upper-earth men shot death into the sea. It was hurt, but not destroyed. The Deep Ones could never be destroyed, even though the palaeogean magic of the forgotten Old Ones might sometimes check them. For the present they would rest; but some day, if they remembered, they would rise again for the tribute Great Cthulhu craved.
