From Lexicanum

The Legio Tempestus was a Titan Legion founded on Mars before the Great Crusade. During the civil war within the Mechanicum in the Horus Heresy the Legio rallied in support of the Magma City, governed by the loyalist Adept Koriel Zeth. Eventually, the Legio found its doom when overwhelming forces of the Legio Mortis attacked the Magma City. The Senior Princeps Cavalerio led his Legio against the Imperator-class titan Aquila Ignis destroying all its retinue of lesser titans only to be annihilated immediatly after by the sheer firepower of the immense titan.

Sources

• ¹ Mechanicum (Novel)