The Unnamed Sentient Mother makes its first appearance in the teaser trailer for The New War, although it may have made even earlier appearances – albeit through a recording – in the Stolen Dreams and The New Strange questline.
Lore
The New War
The Unnamed Sentient makes its proper appearance in the teaser trailer of the quest itself, revealing it to be a gargantuan Sentient Mother, mother to Natah herself. Commanding a massive army of Sentients inside what appeared to be a capital ship or mobile base far away outside of the Origin System, the Sentient Mother commanded Natah to take her "fire" back from the Tenno.
Speculation
Stolen Dreams
As the Sentient Mother shared the same voice as heard in the Arcane Machine in the Stolen Dream quest, it is possible that the audio log are done by the Sentient herself.
Therefore, the Sentient Mother may have made its first, recorded appearance in an Arcane Machine located in a Derelict, linked to an Orokin portal located in a Grineer Settlement on Alator, Mars. After the Tenno uploaded their collected Arcane Codices into the machine, it instead broadcasted an audio transmission with the following cryptic message:
- All-All is silent- Hushed-hushed and empty is-is-is the womb of the sky.
- All is silent and calm. Hushed and empty is the womb of the sky.
The Tenno would then be attacked by an Arcane Boiler accompanied with several Infested units which they fought off, and retreated the vicinity as soon as the Lotus calls the search off.
The New Strange
As the Sentient Mother shared the same voice as heard in the security logs extracted in The New Strange quest, it is possible that these audio logs are done by the Sentient herself.
In which case, the Unnamed Sentient may have made another recorded appearance in the data vault located near the Prison Complex in Nuovo, Ceres. The Tenno infiltrated the vault, and then extracted the security logs in the form of "fragments of a biological signature". The fragments were then scanned by Ordis in the Orbiter, and then plays the security log which featured the sound of the Grineer engaging in a battle, along with yet another cryptic log stating:
- Here we shall search and find. The eyes of day drinking the night.
Later on, Cephalon Simaris decided to help uncover the mysterious message after the Tenno helped him with a few Synthesis. It turned out that the biological signature was Tenno in nature, and – even more curious than before – Simaris then sent the Tenno a blueprint for a Scorched Beacon.
After the Beacon was built, Ordis noted that it seemed to be sending out a signal, which seemed to be directed towards the Arcane Machine they have encountered before. Ordis began to malfunction due to the incoming transmission from the same machine and was intervened by Simaris to save his systems, but the unknown transmission played the following familiar message:
- All is silent and calm. Hushed and empty is the womb of the sky.
The Tenno then tracked down the source of transmission, extracting information about the signal from another data vault in Morax, Europa, to reveal that a Chroma is in the transmission, and that someone is covering their tracks using it.
Simaris deduces later that the 
Chroma in question was not Tenno-controlled, and instructs the Tenno to defeat the Warframe in order to find out the one who was controlling it. However, Ordis, worried about the Tenno Operator instead told them to ignore Simaris' advice and synthesized the Warframe instead, making it impossible to deduce the source of the controller.
Trivia
- It is not explained how Natah has both a father and mother as Sentients reproduce by mean of fission. She is evidently Hunhow's offspring as he could interfere with her, a part of himself, during The Second Dream.
- It is possible that the Sentient Mother is not Natah's biological mother, but rather the mother of the entire Sentient race.
- Presumably, as throughout the New War teaser there was no other thing in the background except an interior of the Sentient Outpost, and then the Sentient Mother appeared behind Natah, it is possible the Sentient Mother was also, in fact, the consciousness of the Outpost itself.
- During the Art Panel of Tennocon 2017 , it was hinted that the original Sentients sent from the Origin System were in fact one singular, massive individual, which fragmented into pieces along the way. It is possible, then, that the Sentient Mother is in fact "mother" to the entire Sentient race, the original progenitor of all current individuals.
- At one point in Chains of Harrow, Palladino refers the Lotus as the "Great Sentient Queen."
- During the Ropalolyst boss fight, The Lotus said that her mother (likely referring to the Sentient Mother) was a carpenter. This could possibly allude to the mother being a producer of housing for the Sentients. However, it is unknown if The Lotus is referring to the Sentient Mother, nor is it confirmed if this statement is merely a metaphor or not.
