Forced regen missions were a type of military operation used by the Great Houses during the War in Heaven, involving forced regeneration.
Early in the War in Heaven, the House Military had learned to "prime" their soldiers, both Homeworlders and recruited from the "lesser species", so that each of their regenerations would result in them becoming increasingly inhuman, taking control of their new forms away from them. The natural endpoint after enough deaths and renewals would be a complete loss of humanoid form in favour of living, biomechanical war machines. By the Third Wave, forced regen missions, where an entire squad was forced to regenerate into forms adapted to the specific terrain before the mission even started, had become a reality. The forced regen missions were viewed as scandalous by the troops themselves, and widened the schism between House Military troops and the Great Houses back on the Homeworld, who showed little understanding of why the warriors resisted the idea so much. (PROSE: The Book of the War)