The Lethean Campaign was the last military campaign of the Second Wave. It took place early in the War in Heaven on the planet Lethe, and it was the last-ever use of casts in a military operation.

With promises of pardons and new regeneration cycles, the ruling Houses lured renegades to the Homeworld. The renegades were then betrayed, overpowered, lobotomised by military psychosurgeons, and fitted with neural links to the remaining babels.

The babels were then released on planet Lethe, where mercenaries and auxiliaries fought for both sides. There, the babels were supreme weapons, committing ruthless massacres against enemy forces. However, an unknown party boarded the timeship where the lobotomised renegade operators were hidden, outwitting both the guards and the security systems before painlessly killing the operators and leaving the babels momentarily defenseless; the babels were immediately slaughtered by waves of enemy suicide attacks.

The Book of the War said that, while some had speculated that the assassin was working for the enemy or Faction Paradox, the most persistent rumour was that they were another renegade, providing the other renegades with a mercy killing. (PROSE: The Book of the War)

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