Velber
- Harvester Star
- a comet shaped like a teary eye that appears in the Milky Way every 14000 years
- a different type of observation device left behind by the alien civilization that created the Moon Cell
- every passing, it destroys intelligent life
- last time it passed over Earth, it wiped out 80% of the Moon Cell's data
- has a core planetoid and several ships surrounding it like armor
- the ships are various invasion weapons
- Altera, the White Titan was one of 3 ships that fell to the Solar System 14000 years ago
- she has a "brother" and "sister" but they're not in this story
Anti Cell
- vanguard sent by the Velber
- organic lifeform designed by the Void Cell to destroy civilization efficiently, like how high-level information lifeform (Servants) are designed based on Earth cultures by the Moon Cell
- every Anti Cell has a different approach, but all grow by absorbing spiritron from intelligent beings and have no sustenance once all are destroyed
- harvestors that work purely for the Velber's directive
Altera (Moon)
- the mysterious Saber that appeared from the unknown region
- main heroine of Extella, the 3rd servant you have
Altera (Earth)
- the backup brain of the Attilaight Cell that destroyed the surface
- discovered sleeping in the remains of the giant by the Huns and raised by them
- she became Attila the Hun after that (ie. this is Grand Order Altera)
Attilaight Cell
- the Anti Cell that fell to the Moon after being purged by the Velber
- the "Titan" Anti Cell that absorbs mana from the civilization she destroys and uses it to grow bigger, designed to destroy itself once the task is done
- she invaded SERAPH as an Anti Cell, and also sent her own data to Earth to create a material body (avatar), invading both at the same time
- did huge damage to both, but was beaten by the Moon Cell on the Moon and by the wielder of the holy sword on Earth
- on Earth she became a corpse, but on the Moon she was sealed since Moon Cell had no way to wipe her out
- Altera (Moon) stayed in the Star of Tears (her ship?) which became the separated "unknown territory"
- 14000 years past and Altera wakes up as the Velber approach the galaxy again[
Bonus:
Altera art from the Fate/Extella Material book. Also sort of spoilers.